And then one soldier comes forth with a light anti-tank missile launcher and settles the fight then and there.
It's worse if these giant reptiles unfortunately find themselves trekking through urban environments, which is where most armored vehicles are at their weakest on account of the prominent number of areas from which ambushes can be launched from, the ever-present threat of concealed mines and IEDs, and the blind spots a bulky tracked vehicles is naturally afflicted with.
The giant reptile isn't a tank, but it's still ungodly huge and therefore is bait for every bog standard trooper with something heavier than a battle rifle. It's not going to have the same amount of armor a tank has either because of the shape of its wearer and the mechanical problems that come along with elevated degrees of weight.
I'm not trying to slam your idea, but massive aliens like that being conquerers doesn't make much sense. Their size is a horrible (and easily exploitable) disadvantage and the ten-footers aren't going to fair much better either.
O_O I didn't think of an alien soldier using an anti-tank missile launcher to deal with a single Zargan >XD I mean, yeah Zargan soldiers have energy shields and strong armor, so, it's possible a lone Zargan soldier could survive an anti-tank missile launcher blast or accidentally tripping a mine and continue to fight after that, but I don't think Zargan would stand still and let the missile hit them, and theywouldn't be alone, the make sure to fight it coordinated groups =3
and yet again, they wouldn't put them selves in a position to be boxed in like walking in a small urban environment, that what the Sas-Zak do, because they're smaller =3 they also have the Sparks which can be uploaded into their armor which could help guild them through dangerous areas =3
and I get what your saying, the bigger they are the harder they fall, I knew that it was going to be a problem, but I thought their technology and diversity would that burden off their back =3 oh, and the Zargan's don't set out to be conquerors, if there is a chance for a peaceful coexistence they'll gladly take that over war =3