Keep in mind, if you master an All materia, you can sell it for 1,000,000 gold. Most people who play FF7 (who know about this) generally level an ALL materia as quickly as possible and sell it to pay for all the breeding, that way you can get knights of the round early, that way you can level the summon up as you play so that when you hit the end game, it's level 2-3 or so, that way you can cast it more than once during a fight.
In your case, I'd either level the summon up, or try to beat the boss with it (it'll help a great deal even casting once).
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