hug the chainsaw to do maximum damage (on yourself)!
His cybernetics were mostly made with the intent that he's going to be stacking more armor on himself and carry heavier weapons, so he's tougher than your average human. He's an inferior model since, aside from what is essentially a greater carrying capacity, he doesn't have many other bells and whistles. Even his strength is mostly just an application of being really tough and making it go as fast as he can in the direction of something he wants to hurt. His radar itself is more or less just a satellite image he's ripping off the internet, since he effectively has a phone built into his head, and night vision which has been mechanically carried over from his life as a faunas. Honestly, he couldn't even pass as a regular faunas considering how bulky and misshapen he looks, but he gets by anyways simply by not bringing it up. When family day comes along this would be very notifiable, as his family look proportionally human while Gren is build like a space marine in full regalia.
The fact he's so heavy and dense is why he can't swim at all, such as the last mission were he was either reliant on his semblance to essentially turn water into a solid surface for him to walk on, or use a whole bunch of external devices to move him through the water in a very sluggish manner. The guy is easily 300lbs+ even without armor, and with his weapons and everything he's a little close to 500lbs total. So you can always try to use his weight against him, like leading him to an area where a lighter person could cross safely, but a heavier one would struggle. His size is, of course, another issue, since he has difficulty moving in small spaces. While his chainsaw at least allows him to do some melee as long as he can press it against the enemy, fighting him in a place where there's a lot of things in the way will limit his options considerably.
In a non combat situation, there's also a matter of self-sustainability. To put it simply, Gren can't get healed. He has to be repaired. He has an artificial blood or sorts that can deal with cuts on his skin, simulating bleeding and clotting, but actual damage to his body can't be healed with rest or regeneration, unless it works on mechanical things too. While he has an indefinite power source, if something is strong enough to damage him and he can't reach somewhere where he could repair himself, he's stuck with that injury. A broken wrist needs to be replaced, or at best jury rigged, but it won't repair itself no matter how much time Gren gives it. And while he has no problems operating during the day, and his helmet tends to dim things if they're too bright, if he's activating his darkvision you can blind him pretty badly by flashing something really bright at him. You'll know he's using his darkvision because it's really dark around him.
His rockets boosters also use it's own fuel too, and while he's going to upgrade that once he figures out how to use his aura to power a jetpack, they need refueling after extended use. Eating food helps but only a little bit, even if he eats a lot. He typically needs a very special type of biofuel he has to make himself, and unless he spends a few hours stuffing his face, once he runs out he's out until he refuels. And the fastest way to do that typically involves opening up his chest and filling up the tank, which removes all those armor bits he's been using to protect himself and expose all the important bits that keep him alive.
Again if you're looking for some sort of kyptonite or a sweet spot that'll kill him dead, it's nothing that wouldn't work on others. Aim for the face, stab him in the back, go for his arm pits, kick him in the dick.
I've always imagined that without the bit of machinery made specifically to channel/create aura ala what Penny can do, his Aura's protective ability would only be limited to the fleshy bits about him, which are really just a few organs and most of his brain.
We know Penny is capable of producing aura, but what we haven't figured out yet is if she could manifest a semblance.
Now what ever indicated that Penny was a prototype? For all we know she is the result of testing from prototypes, which presumably is what Gren is. As I've mentioned before, while Gren is not primarily robotic he is a cyborg, not an android. He was once a faunas and simply started replacing parts of his body after getting more and more injured. Penny on the otherhand is fully synthetic i.e. was never born conventionally.
The main difference is most notable with the machine that helps Gren work his aura. On it's own, what he uses isn't enough to produce aura, so he feeds it his own so that his aura can be used throughout his body. Conversely, whatever Penny is using is powerful enough to produce it's own aura, instead of being fed by an already existing aura. Not to mention that even if we were to use what you're suggesting, Gren still would be actively pumping aura through his body the same way he would be actively breathing; it's just something he'd always be doing. More so since Gren actually relies on his aura to power his body. The only reason he eats and such is both for the taste and to create biofuel for his thrusters, which isn't necessary for him to function anyways.
Of course, then we'd have to debate what is more effective armor: Aura or one and a half inch thick armored flesh.
And what makes you think she was being tested at Beacon at all? Firstly, she was never enrolled at Beacon, she was only there to participate in Vale's tournament. At best this is testing her capability as a combat android, if not also being used for an ulterior political motive. And it's still vague as to what she was being tested for, if she was being tested at all. Nothing in the show or supplement materials suggest that she wasn't a finished product, nor a prototype of any sort. And it's that vague idea of a robot capable of using Aura that I created Gren from.
And if you're saying that objects can't be powered by Aura, then you're also contradicting anothering thing you told Ryo a long time ago when you were arguing about how Ebon is controlling his R-Types. You said that he could use a device that interacts with one's aura, so I imagine that Gren is working off a similar devise, only his aura interacts with said device to power his entire body as oppose to remote drones. You can't just retract that statement now just because Ebon isn't in play anymore.
Also just because it's powered by aura doesn't mean suddenly everything else is obsolete. Because as I said, what Gren uses merely uses Aura, it cannot produce aura on it's own. While it can take aura as a power source, that doesn't mean suddenly all other forms of energy is useless because there is a price to use aura powered machines, and that price is that it needs a person to power it. Gren's machine can be powered by himself since he only needs it to power his body, but you can't use one person to replace an entire power plant meant for a city. Unless you're going to start using people as human batteries, aura powered machines on a large scale is impractical and possibly inhumane. It's not as if you can just take aura in a raw form and exchange it with others, not unless you have the same machine that powers Penny. Which while is still a mystery to us, we can be certain that it's not common place and Atlus may be the only ones who have access to such a thing.
And it all returns back to what I keep saying: Gren, despite all his bells and whistles, is a fairly lowtech version of what Penny is. She is a machine that can produce her own aura, as well as other additional abilities including strength that could match Gren (He could probably stop a truck too if he saw it coming) or even greater (Gren likely can't pull down Bullheads even if he had a hold on them. Shoot them out of the air, sure, but he can't do this.), all while being much, much smaller then Gren, meaning that she's achieving the same level of power Gren can muster but with far less tech to accomplish it. As I said before, the only real benefit Gren's cybernetic body grants him is the ability to carry heavier armor and weapons. His thrusters were not an original part of his body, which he only added after earning enough credits in this RP to pursue that upgrade. And that is specifically not powered by aura, but a biofuel he has to make himself.
And if you're questioning why he would use something he doesn't understand, you could say the same about using cars. If my car breaks down I have no idea how to fix it myself, and I'd have to take it to a professional. Someone does, just not me, and as long as it doesn't breakdown I can continue to use it to the best of my abilities, and overtime figure out how to work out any errors short of total destruction. In the same respect, Gren's body is like a car. He knows how to drive it, and has a workable knowledge to improve and repair some of it. But if he fucks up so badly that he breaks something very critical, he's going to need professional help. And unfortunately for him, the only people who would likely know how to fix him are either the White Fang, who stole the technology he's made from, or Atlus, from whom the technology was stolen from in the first place. And it's all the more reason why he continues to layer armor and aura over himself to minimize the damage done to himself, as well as trying to improve his mobility to avoid damage all together.
I just had the best god damned waffles of my life.
That's assuming that just because Gren lacks most of his original body, his aura is suppose to be weaker. This isn't Star Wars where cybernetics are inherently conflicting with mechanical parts or even armor. Besides that most of his aura already goes into making the machine work anyways, and that isn't limited to just allowing him to move. It effectively pumps his aura throughout his entire body like a mechanical heart, precisely so he doesn't need to constantly focus on pumping his spiritual blood all the time. With the benefit of doing what aura is suppose to do anyways, like protecting its person.
Gren's aura is constantly active for both protecting him and allowing him to live much like Sapphire is constantly breathing. Because as discussed before, Gren's aura is what is powering his body (since we also nixed the idea of alternative energies like dust or fusion cores). We also had mentioned that while EMPs are effective against Gren, it's not instantly lethal, only that it shuts down a vast majority of his functions beyond what's keeping him alive. I.e. Moving at all.
The idea that he's constantly struggling to make his heart use his aura to just keep him alive is frankly kinda ridiculous,
...not to mention making him much more of a liability If he can't rely on his aura to serve a basic function everyone can do.
If you want to nerf him that badly, I'm willing to satisfy that desire by limiting his semblance instead, shortening its range and adding more caveats to avoid being affected by it.