Cold sweat.

I like to use this technique not just to heal or become invulnerable, but to waste time. Most times when people see Mei they try to wait her out and attack when she emerges, so this gives my teammates a free target to use an ultimate or blast them away. And once they're blown apart, then I can pick off the ones left and advance. So if you se Mei trapping herself, lend her a hand, because *Fuck the Red Team!*