As the title suggest, I would like to know how to fully grow a Enemy Skill Materia: I managed to get some E.S. but not every fight brings me one. There's a strategy to get 'em all?
E Skill learns new skills by being subject to those skills in battle. So it's a matter of fighting the proper enemies, not enough battles. It's also impossible until the end of the game, due to one being in the last dungeon. There are 3 "problem" E Skills, as it were, that cause some frustration. 2 because they can be lost forever and one that's fairly luck based.
Past those, the rest tend to be a simple matter of fighting the right enemies and waiting for the right attacks. There are some buff spells which will need you to use the Manipulate materia on the enemy, such as Dragon Force, Angel Whisper, and White Wind. If you see an enemy using one of these skills, then use Manipulate and force it on yourself if it's available. Past that, the rest is all a matter of getting your face beaten in.
General strategy is simple - always keep that Materia on at all times! Then you will not miss the one-times and you'll always catch it when the attack happens in normal scenarios, rather than having to hunt the skill down amidst all the rest of the late-game stuff you need to do.
You might be interested in knowing that there are four enemy skill materia available in the game. Their locations are, as listed on this wiki:
If you want to max out all four materia, you need to have them equipped by a character who the enemy skill is being used on, so in the case of getting Trine on all enemy skill materia that don't have the skill, from Godo, after already beating Materia Keeper and Stilva, you have to have all those enemy skill materia equipped by Yuffie.
I always kicked myself over not getting Trine on the fourth enemy skill materia well after getting pretty much everything else in the game. My save file will be forever tainted.