quote Quetzalcoatl
You already played the mission that gives you the Item Fusion Tomb? Just for you to know, you can also use items to fuse your materia and given you stat pumps.
Anyways, fuse Curaga with your HP Up materias. hell Thungaga/Dark Firaga are good to spam, so have those for now. Octaslash/Assault just increases your chances on triggering that DMW so only equip those to unlock trigger cutscenes to compelte your DMW. Right now just worry on having a Curaga with HP boost and Thundaga/Firaga with magatk boost. Play more missions so you can unlock the limit break items, that will make you stronger.
Quetzalcoatl: Dude, it's called Item Fusion TOME, not Tomb. Item Fusion Tome. No one would wanna go to a tomb and do stuff... unless you want dem Genji gear, lol..... I hate those later missions. T^T
Some advice from me is to fuse some of your stat modifiers to some of your spells that you use the most. A REALLY good idea besides HP stat modifiers is to fuze VIT (Vitality) modifiers. This will increase your defense against physical attacks, and as most enemies in the game tend to be physical attackers, increasing your VIT is a good idea. Use some Materia with a pumped up VIT modifier and use Barrier... you're looking pretty sweet then, lol. The Materia is up to you, but the smart thing is to use the Materia that you consistently use. That way, you can fight at your regular setup and style, and still be a badass.
...Oh! I meant to mention one more reason for applying VIT modifiers. There's these enemies later on (I think they're called Homunculus) that can slash at you from a distance when they attack. They won't attack you up close all the time, but they have this cheap little thing where if they swing at you, their attack seems to be sent right up in your face instantly, and you'll be fighting more then one in the later missions... so VIT is important! You can increase your Spirit stat if you want, but I seemed to always encounter more physical attackers than magical ones. Just using a mastered MBarrier (or Wall) should suffice imo.
Also, watch out for Death and Stop. Seriously, knowing which enemies can cast/inflict those and equipping Anti-Death/Stop items is a Godsend, because if you get Death, you die, and if you get Stop... well you're just screwed then (unless you have high VIT lol).