Ishtar

A single merge won't really change her matchups that much. Personally I'd save her for Odd Attack Wave, considering she's the only unit atm that has it and there are a bunch of units that would appreciate it, like Lilina, blade tomes, brave weapons, etc.

If you want to eventually +10 her, merge. Otherwise, Odd atk wave fodder. But keep one with good IVs, she is awesome.