I was using OGS-019 Master Yi, and I had a Stalwart Poro on a battlefield. Enemy had a Ravenbloom Student and an Eager Apprentice in the base. It was the enemy’s turn. He moved the Apprentice to my battlefield and started a combat showdown. He started a chain with Smoke Screen targeting my Poro. I passed and let it resolve. Then I started a chain with Discipline and it resolved. He passed focus and I casted Rune Prison on his Apprentice, he reacted with a Stupefy on my Poro. And the showdown ended with no unit died, and the Apprentice was sent back to the base. What should be the might of my Poro at this point after the showdown? He then attacked again with the Ravenbloom Student. What should be the might of my Poro when the combat showdown started?

After the first showdown your Poro is at −1 Might (functionally 0), and it's back to 2 Might the moment the Student's combat showdown begins.

The key is Smoke Screen's snapshot. When it resolved, the Poro was already the Defender in that combat showdown, so it was 5 Might (2 base + 2 from Wuju Bladesman while defending alone + 1 from Shield). Increases are applied before reductions, so Smoke Screen snapshotted the full −4, not −1 — the "to a minimum of 1" clause only limits the reduction at the moment it's applied (Rule 477.3.b, FAQ #6530).

After that you get: −4 (Smoke Screen) + 2 (Discipline) − 1 (Stupefy) all persist for the turn, while the Shield +1 and Yi +2 are passives that switch off the instant the combat/Defender designation ends. So post‑showdown: 2 + 2 − 4 − 1 = −1 Might — it's treated as 0 whenever a spell/ability references it or damage is summed (Rule 143.2.b), and it doesn't die since Combat Cleanup healed it and it has no damage marked.

When the Ravenbloom Student moves in and the new combat showdown opens, the Poro is a solo Defender again, so Yi (+2) and Shield (+1) come back online: 2 + 2 + 1 + 2 − 4 − 1 = 2 Might at the start of that showdown.