I probably learned half the shit I know about this game from these 2 links.
Assists come out at different speeds so it will vary based on your characters assist.it also depends on what your trying to combo into the assist.
example : psylockes aa assist comes out fast so if your magneto you can go cr.lk, cr.lk psy AA by pressing the first cr.lk then pressing psylocke assist button, then cr. lk and it will combo.
on the other hand, sents rocket punch asssist comes out slower. youd have to press either his assist right before the two cr.lk’s or press his assist the same time as the first cr.lk and time the second cr.lk a bit late. I think most people do either or but if you wanna hit that cr.lk at the beginning of the match you should try the 2nd method.
check out higher jins guide here for info on fast-fly, the unblockable, and unfly http://www.shoryuken.com/showthread.php?t=95138&page=1
basically these are the essential guides to play a competent sentinal. maaybe unfly isn’t as important, but its good to know.
accessible characters: storm, sent, cable, mag,( <- AKA the four gods of the game. if you dont have one of these characters on your team, then prepare to work hard as hell) iron man, cyclops, psylocke, strider w/ doom assist, blackheart… you can definitely make other characters work but at med to high level play your gonna see these characters a LOT.
“accessible” assists (lol): capcom B, Iron Mans B, cables B, Cyclops B, psylocke A, Dr.Dooms B, Storms A, Sents A(rocket punch) or Y(drones) tron bonnes Y. you can use others but I think those are all the commonly played ones.
most ppl just play common teams like:
mag/storm/psy or MSP
mag/storm/sent or MSS
mag/storm/tronne or MST
storm/sent/capcom or team santhrax
sent/strider/doom or clockwork
MIP: mag/Iron man/Psy
there are others but basically those teams are the best as far as team dynamic. check the wiki for more.
oh and yeah watch LOTS of videos. they help like no other. I learn/understand new things almost everyday when I watch a new video. youtube.com and zachd.com have tons of MVC2 videos on them.