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If I want to increase a summon card which summon 2 or + monsters (as "shadow wolfs"), did I have to pay the additional cost for multiple targets ?
First of all, you might want to consider using spoiler tags. It's only a mild spoiler, but some people might be vexed.
Second. I don't think setting base stats for summons is considered multi-target since dropping a summon doesn't "target" anything, in the first place. I'd see it being an issue if you were increasing attack on a summon with attack +X, target 2+, though.
Have you checked the class-specific FAQ section?