If Nidalee might 5 stunned and -2 might by defaint dance in combat with Stellacorn might 3 +2might by defaint dance as well. Who win combat?

Stellacorn's controller wins — Nidalee is killed and the combat is not a tie.

Here's the breakdown:

• Nidalee is Stunned, so she contributes 0 might to the combat damage step (Rule 423.1.b). Her current Might (5 − 2 from Defiant Dance) doesn't matter for dealing damage.
• Stellacorn (3 + 2 = 5 Might) assigns and deals 5 damage to Nidalee.
• Nidalee's current Might is 3 (or even her unmodified value), so 5 damage is lethal — she's killed during Combat Cleanup (Rules 423.1.c, 461.1).
• Stellacorn takes 0 damage, survives alone, and is the sole unit remaining → Stellacorn's controller wins. No attacker recall happens because no attacker unit remains, and Nidalee's "win a combat" draw does not trigger since she did not remain (FAQ #10708 logic).

One caveat: if Nidalee had instead survived (e.g., she took only 3 damage), this would be a tie and the attacker's units would be recalled (FAQ #3527) — but at 5 damage to a 3-Might unit, she dies regardless.