This is a boss fight
The fight is in 2 phases.
1st Phase you will fight him in a big circle room and will perform a variety of attacks.If you can keep the BFG ammo for the 2nd phase has they are more usefull there
1. Normal Rockets fire. He will fire 4-5 rockets (one at a time) from is left hand.Just move left or right to avoid them. They are pretty slow and easy to avoid
2.Mortar Shot. He will sent a volley of rockets in the air. Look on the ground and stay out of circles
3. Charge Shot. A big glowing orb will appear on his left hand. At the end he will fire a very quick laser like shoot. You can avoid it by moving out of the way at the last moment or prevent him to shoot it by either doing enough damage to him or shooting the canon. If you prevent him from shooting, he will be knock back a little and will drop ammo and health.
4. Charge. This is a normal charge. Nothing special about it. Just move out of the way
After beating him, a small cutscene will trigger and he will revive for one more round.
In this phase he has the same attacks has before plus two new ones.
5. WallsHe will create two walls on your left and right forcing the fight in a corridor. This attack does no damage but considerably shrink the room you have to move. He will not do all of his attack during this time but he will do his normal rocket shots and the new attack of this phase (Blast Wave). So far i only saw him do these attacks while the walls are up. Also the BFG is very usefull here since it breaks the walls when it detonate.
6. Blast WaveHe will sent 2 Blast wave your way. One at leg level and the other at head level. You jump over the low one and duck under the high one. It is possible to dodge them by moving left or right but this will require you to be at a good distance from him since the wave are quite big.
In both phase he has the same amount of Health.