Rite of Passage

Whenever a creature you control is dealt damage, put a +1/+1 counter on it. (It must survive the damage to get the counter.)

Hardened Scales

If one or more +1/+1 counters would be put on a creature you control, that many plus one +1/+1 counters are put on it instead.

Lost Jitte

Whenever equipped creature deals combat damage, put a charge counter on Lost Jitte.
Remove a charge counter from Lost Jitte: Choose one —
• Untap target land.
• Target creature can't block this turn.
• Put a +1/+1 counter on equipped creature.
Equip {1}

Strength of Will

Until end of turn, target creature you control gains indestructible and "Whenever this creature is dealt damage, put that many +1/+1 counters on it."

Michelangelo, Weirdness to 11

When Michelangelo enters, create a Mutagen token. (It's an artifact with "{1}, {T}, Sacrifice this token: Put a +1/+1 counter on target creature. Activate only as a sorcery.")
If one or more +1/+1 counters would be put on a creature you control, that many plus one +1/+1 counters are put on it instead.

Walking Ballista

This creature enters with X +1/+1 counters on it.
{4}: Put a +1/+1 counter on this creature.
Remove a +1/+1 counter from this creature: It deals 1 damage to any target.