Derevi, Empyrial Tactician | Cauldron of Souls | Aerith Gainsborough (and one other card)

Cards

All permanents on the battlefield (
Derevi without a -1/-1 counter on it and
Aerith Gainsborough with a +1/+1 counter on it).

Activate
Cauldron of Souls by tapping it, giving
Derevi and Aerith persist until end of turn.
Activate
Evereth by sacrificing
Derevi.
When
Derevi dies, its persist ability triggers, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
When
Derevi enters the battlefield, it triggers, untapping
Cauldron of Souls.
Resolve the
Evereth ability.
Activate
Evereth by sacrificing Aerith.
When Aerith dies, it triggers twice.
Resolve Aerith's persist trigger, returning it from your graveyard to the battlefield with a -1/-1 counter on it.
Resolve the second Aerith trigger, putting a +1/+1 counter on each legendary creature you control, thus removing the -1/-1 counters from Aerith and
Derevi.
Resolve the
Evereth ability.
Repeat.

The combo does not put +1/+1 counters on
Derevi or Aerith, but does on any other legendary creature(s) you control.
If you have a way to kill
Evereth and ({1} magic symbol) ({BR} magic symbol) available, you can do so after putting a significant amount of +1/+1 counters on it to deal damage to each opponent using its second ability.

This combo has been estimated to be Exhibition (Bracket 1+).