Personally, I think my water team is pretty stacked (Holy Trinity + Ryu-Oh) but Tiamat is a godsend, even more for Water that lacks Damage, double Tiamat has mathematically a tiny damage edge over double Reiki but still, I'll be using double Jack Frost for Fire content and you most likely should to...

For me the fact that I can blast through Accessory Quest 4 with my Water Team at 7000 HP is kind of mind boggling, even more so the fact that I do this and always finishing at 90+% HP, most of it comes to Jack's Double Damage Resist, so it's nothing to sleep on, personally.