if my oppponent has a 8 might unit on the field and then plays sky splitter to reduce its cost to 0, since it cost 0 for my opponent, can i defy this now?

No, you cannot use Defy on Sky Splitter in this situation.

According to RiftJudge FAQ #4278, #3402, and #6358, Defy checks the printed energy cost of the target spell, not the actual cost paid after reductions.

• Sky Splitter's printed energy cost is 8.
• Defy can only counter spells that have a printed energy cost of 4 or less.

Even though your opponent paid 0 energy to play the spell due to the reduction from their unit, the game still treats Sky Splitter as an 8-cost spell for the purpose of effects that check its cost, such as Defy. Therefore, it remains an invalid target for Defy.