It is a widespread belief that dungeons possess some kind of intelligence. From taking monsters away from dungeons and observing their behaviour away from their parent dungeon to the way their layouts are set up (especially on subsequent runs of any individual dungeon), it seems pretty likely that there is some kind of mind running the show, making decisions about what to do and how.
And this is the reason why the concept of a "tank" makes sense. If the monsters were mere animals with the level of intelligence they demonstrate on their own, they would probably just attack all members of a raiding party, but the dungeon does notice that there are often advantages to focusing on that one specific party member: they are the anchor to a handful of buffs that, should they no longer be in effect, would make taking out the other party members a lot easier. So the "standard tank stuff", here, consists of buffs that redirect most damage Elisabel and Tom take to Chris, and also reduce the amount of said damage that they take in the first place. They're reasonably MP-efficient, and their main cost is in the tank's lifeforce when active, so tanks don't need to invest too much in MP for them.
There are other, more specialised tank skills that depend on the specific situation, but these buffs are pretty generally useful for the vast majority of dungeons.