I built my Finn (yes, Finn) with Harmonic lance+ (+3 speed).
She reaches close 40 speed with a single +3 speed seal.
But Im not sure which is the best special for this kind of weapons. My opntions:
a) Moonbow and Luna.
b) Glimmer
I tryed Moonbow and Luna and I did not see the damage was that high... Im not sure if I should try Glimmer or?
They are good for different things. Moonbow and Luna are if he's having trouble with high-defense units, whereas Glimmer is good for lower defense units. If he has trouble with Armors or Dragons, Moonbow/Luna is better, if you just want extra damage overall then I'd go with Glimmer.
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