I'm not sure if this is already know, but from I what I've read it seems like it's not, so I guess I'll let everyone know.
I've played FF7R and I decided to play the original FF7 and Crisis Core again, and I just found myself doing the ASAP challenge. I'm at the mako reactor 5 and after beating Minerva I decided I wanted to get all the materia I wanted before proceeding as well, and I thought it was impossible to get a HP +999% Costly Punch at this point, but I tried it anyway, and as it turns out, it is possible.
I'm not entirely sure exactly what has to be done, but I did it kinda like this:
Lv.1 Goblin Punch (ATK +1) + Lv.1 Murderous Thrust (HP +20%) = Lv.1 Costly Punch (HP +20%)
Then, I leveled up Costly Punch to level 2, but it remained with HP +20% and I couldn't fuse it with anything to increase the HP bonus, so I went ahead and leveled it up a bit more. Fortunately, I got help from the moogle and it mastered my Costly Punch, but it still was only HP +20%. But then I bought 3 Libra (ATK +1) materias from the shop and fused it until my Costly Punch had HP +50% (yeah, I thought it wouldn't work too). From here, I just bought 95 Doublecast materia from the shop and fused them until I had my Costly Punch HP +999%.
Maybe if I had leveled Murderous Thrust a bit before the fusion I wouldn't have to level up Costly Punch and use Libras, but I'm not gonna try that now.
I'll list below my good materia so far, but I think now I'm good for the rest of the game.
Curaga HP +999%
Costly Punch HP +999%
Gil Toss VIT +100
Wall SPR +100
Quake MAG +100
Darkness ATK +100
SP Barrier VIT +100
Graviga SPR +100
Darkness HP +999%