Protip: weaknesses of Ulysses include:
Electric shocks.
Water.
The backs of the knees, the neck, the armpits, the ankles, and the base of the spine are all poorly armored. Wedging a sharp thing in there can do damage.
Not really a weakness, but it's worth noting that the reactor core, despite being a giant glowing light, is the most heavily armored part, tied with the back.
Blunt force trauma to the head can disorientate him while he wears his helmet.
He is far less accurate without his helmet.
He is less perceptive in general without the helmet.
he is slow in his armor if he isn't dashing or jumping. However, both of those require commitment I.E, once he decides to dash in a direction he's stuck doing that.
He has a hard time turning all the way around. 
He is very heavy. This can come in handy, be creative.
If someone is on his back, he will have a hard time removing them. 
All his weapons take time to transform, and only his homing rockets or machineguns are good against aerial targets. Get in the air if you can. 
His large size prevents him from being all that useful indoors. However, do not get stuck in a tunnel he can fit in, you can't fight him head on and have much of a chance.
Most of his weapons have long barrels, meaning if you get close enough he cannot shoot you.
Engaging him at long range is suicide if you have no cover. His sniper rifle can explode tanks. Meat won't fare better.
Do not underestimate him unarmored. If you destroy his armor, he is much faster and skilled at hand-to-hand combat. He loses all ranged capability however.
Cold slows him down, as well as force him to recharge slower. use if possible. 
Hiding in buildings is a bad idea, he is quite good at demolishing structures. 
If he's somehow allowed to use it, his nuke can be caught and thrown back at him, destroying his armor instantly.
Try plugging up his flamethrower with something. it's hilarious.
Alternatively, try turning his helmet around.
As long as you can keep out of line of sight and get close, he's not that threatening.
If you can get him to shoot at you enough, he may expend all of his power and drop to a knee to recharge. He is most vulnerable then, but remember he's aware of this.