If the creature that triggers Mirror March is disabled you get to keep all reinforcements

soultwist

Cast any creature: lets choose Zacama, Primal Calamity (just to get some red gems)
Cast Mirror March
Cast Alchemist's Vial on Zacama
While Still disabled, and with 9 or more red gems,  cast a 2nd Zacama.

Because Zacama is disabled no gems will change color on ETB effects.  Mirror March will start reinforcing.
You will end up with some number of reinforcements X, say 108/108 Zacama
Zacama will not attack (disabled)
Turn ends
You now have a free and clear 108/108 creature

jtwood

I’m okay with this not being a bug

FindingHeart8

Yeah let's fix the hundred or so other glitches harming player gameplay then maaaybe look at this one maybe