This is regular Van Kleiss, not Giant Van Kleiss. At the start of the battle make sure you have full omega energy and your best omega build equipped. Unleash your omega build on him until he jumps to a higher ledge. He’ll start shooting projectiles three waves at a time. Stay at the back of the stage and maneuver through his projectiles. When he takes a short break, shoot at him with the Slam Cannon. This will cause him to eventually come back down. If you have enough omega energy, finish him off. If not, avoid his attacks and kill the crab guys until you have enough.