The first boss fight is basicly an exercise in using the lock on feature. Using the two small center blocks for cover, most bombs can be easily avoided. Just keep their explosion radius in mind. Xedur rarely drops bombs in the same place twice as he's constantly on the move. Whenever you see an opening to stand still for a couple of seconds, lock on and see if you can get a shot in while locking Trace in place. Xedur's armour is impenetrable by the Axiom Disruptor.
Use the small platform and switch between jumping over his low shots, and ducking under his high shots. Face right and take potshots at the growth on his back while doing this. Prioritise not getting hit over damaging Telal and stick with the pattern, which will slowly speed up as Telal loses health.
Alternative strategy: Walk straight into him and keep pressing right to get past Telal. From there he can't hit you at all, and you're free to quickly finish him off from behind.
This enormous boss is actually pretty easy once you figure out the "sweet spots" in the level from which Uruku can't hit you. The first is on one of the pillars where the pillar in front of you blocks all of his shots, including his giant laser! Take out all of his top weak spots from there, including his laser cannon, to open up the second sweet spot.
Without his laser cannon, Uruku can't hit you when you stand on the leftmost tile of the small platform above the entry. From there you can hit his lower weak spots with Nova, as well as his face whenever it's exposed. It will take a while, but stick with the strategy and you'll be pretty much invulnerable throughout the whole fight.
Shoot it. This isn't even a boss fight - it's a literal abortion. Trace will express some moral concerns after this event.
Ukhu is arguably the hardest boss in the game. Just like in the Gir-Tab fight, there are no hiding spots whatsoever, and his pattern makes him hard to hit. You can only get hits in when he opens his mouth. I found the Voranj to be the easiest weapon to actually hit him with in the small openings Ukhu gives you. There is however, a trick to delivering a massive amount of damage in a single hit: you can glitch out the smaller enemies he spawns with an Adress Bomb. If you are out of bombs it's worthwhile to just try and use the Adress Disruptor and take a few hits to do this. The glitched enemies will drift upward and explode as a group when hit. If you can get Ukhu to line up his head with their position a well-times shot with the Voranj will nearly kill him in one hit. There is a save room nearby so don't be afraid of dying - you'll get Adress Bombs back for free. It's possible to kill the smaller enemies (I recommend the Kilver for this) and beat Ukhu conventionally. However, since he speeds up his pattern, it's better to get a few shots in and finish him off with the aforementioned strategy.