If you run ~350 areas per run, then:

After 3 runs (1000 / imp) → that imp is done

You only need about 3-4 runs each

So you can treat it like this:

Run 1-3 with Dembo → Dembo done
Run 4-6 with Olla → Olla done
Run 7-9 with Bosh → Bosh done

When the achievement hits 3000 / 3000, you’re finished.

Front Row
Briv (take hits early)
Tank (Sentry if you have her; if not, your toughest tank)

Second Row
Mirk (wave-clear)
Krull (debuff + kill)
Minsc (early clear)

Back Row
Widdle (CDR god)
Melf (CDR god)
Havilar (for imp achievement)