This goes for *any* difficult part of the game. If you're finding yourself getting smacked around, go and do a few missions at a save point. Yuffie's ones can really give you some great items and plenty of chances for your DMW to level you up lots.
As for Angeal, Hell Firaga/Thundaga works pretty well in that fight. You ought to be able to do 3000-4000 x 3 damage each attack with Hell Firaga, whchih should be sufficient to kill him on one or two ethers. Or, if you don't have it already, get Costly Punch which can do 9999 or Gil Toss which will do about 5 x 4000-6000 damage - though the latter will send you broke pretty quickly. Otherwise, keep Raise activated (later on in the missions, you'll be able to have an infinite supply of Phoenix Downs, so don't stress too much about using one).
Also consider making a HP Up++ with HP +999% attached to it, and then fusion that to your Hell Firaga/Thundaga. That way you always have an *awesome* attack, AND a stat boost. I would recommend doing the same for MP+999%, Magic +100, Luck +100 and Strength+100, substuting Hell Firaga for the appropriate spell set you'd like to use. Ultima, Costly Punch and Flare are good calls, as is Full Heal or Curaga.