This achievement requires the completion of 12 Variant Adventures.
Variant Adventures are different settings of an adventure with possible changed restrictions, objectives, requirements, and rewards. Once you complete a regular adventure the variants will unlock of varying difficulty. Also, if you are participating in events they have Variant Adventures which count towards this as well.
You should be able to complete any of the ones labeled in the green "Easy" difficulty. Also, the yellow "Fair" can be completed with a bit of difficulty. Anything other than that it will probably be too difficult.
Adventure Variants can involve reaching the same Area or higher, taking up slots in your formation, additional enemies spawning, etc. There are many different ones to look out for.