Whipfist.
Lock onto one of the sentinels and charge. When you're almost within range, do a mid-high jump to either side of the boss and charge the Triangle ranged attack, and then evade. If you've got the HP, do an Air Painspike or whatever it's called; the single-target tentacle rape attack. You hover in the air when using it, so attack with the same method as above.
The boss is actually really easy when using this strategy. It just takes like, 15 to 20 minutes.
EDIT: actually, you're probably in your best interest to use this strategy for basically EVERY boss battle.
In other news, fuck GTA4. I shot one of the pigeons while doing the final mission, and there's a bug in the game where that pigeon won't count to your total if certain conditions are met, which I met, so I was locked at 199/200 pigeons. Lucky me, I had a save file from just before the Deal/Revenge point, so I've gotten all 200 pigeons in my other file, but fuck GTA4. That's 15+ hours of gametime down the hole, at least 12 were pigeon hours. (I've been to every pigeon location at least 4 times within the last week. That's 800 spots.)