Persist

[average_j03]

If a creature has a -1/-1 counter on it and a +1/+1 counter gets added, does it nullify the negative counter? In which case would persist work again? Ie  {Glen Elendra Archmage}

[Dburlewd]

It negates the -1/-1 counter

[D3g3n]

+1/+1and -1/-1 counters cancels each other out, so you could have persist work again. Similar case with Undying.

[scarsabrex]

Just remember in the case of both effects if the creature dies due to a combination of the counters it will not respawn.

[Fishsticks123]

Black Mikaeus + Bloodflow
Connesiur + any persist creature with abilities like kitchen finks.