This is super basic.

After spending time on each character with ignite, I found that using Hybrid was by far the easiest.

1. Go into Arena at 200%
2. Upgrade Hybrid's Skill 2 (Whisper) twice (Ignite on hit, +50% radius)
3. Kite enemies, dash into centre of group, use Whisper.
4. Achieve greatness.

If you don't get it first try, fret not. This should almost guarantee the achievement in under 10mins.