if i use Shuriken Flip on my mournful witness at my base. and target a 2 might poro at a battlefield. it kills the poro and moves my unit. does a combat showdown still occur or is the poro dead before it moves so its a non combat?

No — this is a non-combat showdown, not a combat showdown.

Here's why: Shuriken Flip's damage and move both resolve inside one chain item, so at the moment the Witness moves, the Poro only has lethal damage marked — it isn't actually dead yet. The battlefield becomes Contested (CR 447/450), but death and Combat staging don't happen until the cleanup after the chain resolves (CR 455). During that cleanup the Poro is killed (CR 323.4), so when the game checks CR 323.8, there are no units of two opposing players at the battlefield — only your Witness. Only a Showdown gets staged (CR 323.7), and when the turn player initiates it, it opens as a non-combat showdown (FAQ #11630).

Practical result: no Attacker/Defender designations, no "when I attack" triggers, no Combat Cleanup healing, and your Witness conquers the battlefield when the showdown closes since you're the only player with units there (Rule 348.2.a).