BTW all companions can tank unfair with 80+ AC and a bunch of immunities with the right setup. Seelah, camellia and wenduag/lann are the best bets though, since they only have 1 level at their base class. Camellia is good at this even with her base class, but a tank without any other utility is a no go for unfair imo.
I can get Lann and Wenduag and Camellia to over 80 ac but how do you do it with Seelah? Also Camellia's utility is stabbing people and having frightful aspect so you can get shatter without dirge of doom or a divine caster running up.
With Seelah, you have to make
Desiderius cry
At lvl20 she's : 13 pally/5 alchemist/1 oracle / 1 monk pajama tank
11 pally is base for mark of justice, that was the whole reason I recruited her. Then I gave her 2 more levels to get another greater angelic aspect in the party for cold/acid immunity.
5 Alchemist (chirurgeon or sth) was to get cognatogen charisma bonus for MoJ. The added AC helps, but infinite true strikes/Shield and exp. retreat is the MVP here. Chirurgeon for free skill focus : religion feat
1 oracle for nature's whispers (Nature oracle revelation that gives cha to AC instead of dex to AC). I choose enlightened philosopher to get a free knowledge : world skill focus.
1 monk is scaled fist obviously. She's a pajama tank, but she does an OK job for a party wide +11 AB dispenser.
Camellia is the best tank if you want to roll with base classes for some roleplaying reasons, but other than that frightful presence can be cast on anyone, and she does lack some key buff spells like death ward / life bubble / heroism etc. I prefer brown-fur transmuter tanks to give the entire party elemental and crit immunities, so I rolled Wenduag as one :