Hot Pocket

This is an RNG achievement (especially with multiple people). All you have to do is get thrown into Ignis' slag pot. (Please note you won't be thrown into the slag pot if you have highest aggro e.g. tank)

If you are a monk, you can use Summon Black Ox Statue. The statue will agro the boss and you don't have to do anything else but wait to be thrown into Ignis' slag pot.

This may be obvious to most, but you need to be on Ignis' threat table in order to get thrown in the slag pot. i.e. you cannot just stand there and do nothing while your friend tanks him. So either do some very light damage, or a non-damaging action (e.g. druid Soothe). You'll know you're on the threat table when your tooltip for Ignis shows 0% threat (or more). Prior to that the tooltip doesn't show any threat value at all.