lets say opponent plays Rampage targetting his First Mate and pays the optional recycle to make first mate +2 might. i react with the hidden card "Hidden blade" to kill my unit targetted by rampage. does first mate get the +2 might buff?

Incorrect answer, AI thought it's +2 shield

Yes — First Mate still gets the bonus +2 Might. Rampage's bonus is gated only on whether the caster paid the optional [C] additional cost, and that decision and payment happen while playing the spell, before you ever get priority to react — it's locked in and not refunded (FAQ #12602). The buff instruction only concerns the friendly unit, which is still a legal target when Rampage resolves; killing your own unit with Hidden Blade only removes the enemy half of the spell, so the mutual damage instruction simply does nothing (Rules 359.3.e.5, e.8 — spells do as much as they can). Net result: First Mate gets +2 Might, takes no damage from Rampage, and you draw 2 off Hidden Blade. One caveat: if Hidden Blade was played from its facedown/hidden card, its target must be chosen from units at the battlefield it was hidden at (Rule 811.1.d.2) — that doesn't change the ruling above, unless of course your targetted unit being was in base.