Mix and match these recommendations as you feel necessary!
Have a raid leader or raid-assist using Deadly Boss Mods. It will put a skull on the victim (and announce it to the raid via a colored raid warning).
- Recommendation #1.5:
Have raid icons enabled on your raid frames. I use GridStatusRaidIcons and GridIndicatorCornerIcons to allow my Grid frames to show the raid icons (the graphics) in the corners of each health box.
Add the Slag Pot debuff to your raid frames, if available. Grid users, you can follow instructions here on how to do that. The Slag Pot debuff is also included in the GridStatusRaidDebuffs extra addon. Other raid frames may or may not have automatic adding of these debuffs (Xperl does, and sRaidFrames needs to have debuff tracking enabled for it) .
Use an assist macro and click it whenever Ignis is choosing his target. This requires awareness of when he's charging/grabbing his victim, because once they're in the slag pot, Ignis will target the MT again and thus the macro will fail to target the slag pot victim. If you want to go this route, though, here is the assist macro:
Have them call out in your voice chat/ventrilo. They should say their name, and if your raid doesn't know each other well, their class so that the healers can more easily locate them. "Druid Kae in the slag pot!" etc.
Hope this helps :)