I was using OGS-019 Master Yi, and I had a Pouty 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. He then attacked with the Ravenbloom student. What should be the might of my Poro at this point?

Your Poro is 2 Might when it defends against the Ravenbloom Student.

The key is what Smoke Screen saw when it resolved. By then the Apprentice had already moved in and the combat showdown had started, so your Poro was the lone defender and Wuju Bladesman's "defending alone" passive was live — its Might was 2 + 2 = 4. Smoke Screen's -4 is clamped by its "minimum of 1" clause and snapshots at that moment, so it locks in as -3 for the turn, not -4 (Rule 477.3.b; increases apply before decreases, FAQ #1455).

After that: Discipline +2, then Stupefy -1 (applied unclamped, since the Poro was at 3 then). So when the Student attacks: 2 base + 2 (defending alone) − 3 (Smoke Screen) + 2 (Discipline) − 1 (Stupefy) = 2 Might.

Worth flagging the trap: if you'd let Smoke Screen resolve before the Apprentice moved (as in the classic Watcher/Stalwart Poro line), it would have snapshotted at only -1 and your Poro would be 4 here. Because it resolved mid-showdown, you take the -3.