Basically: -Tap jump just very slightly when jumping over him to not shoot above him. -When he acents, you can get a ton of shots in by standing very close to him and timing your jump well. -In his giant form while he is preparing the bullets, shoot at the peek of your double jump to get some shots in. -Finally you can save some time by jumping at the rocket barrel while it´s decending.

Here´s the video: