I really want to get rid of Seelah - What's a good tanky replacement class, merc-wise? I guess with the power levels being what they are the classic heavy armor/shield approach won't work.
Almost any class can become tanky, if you are willing to use buffs. Since you're playing a Sohei, you can boost your horse's AC to make him unhittable without having to waste a character as a "tank" (your characters should really do more impactful things than soaking up hits). Make him Bulky (or whatever the archetype that gives armor proficiency is called) and cast Shield of Faith, Barkskin, Cat's Grace, and Shield on him. Take Impossible Domain: Nobility on your Cleric for the Insight bonus, and you should be set. So I guess my answer is an Alchemist, since it allows you to cast both Shield and Barkskin on your mount. Make him a Vivisectionist if you also need a melee fighter or an Incense Synthesizer for the alchemical bonus to hit.
Personally, right now I'm using a Bard Seelah as my "tanky" character. With a light armor, a shield, the Dodge feats, and Crane Style, she is sitting on AC 40 since level 6. I'm playing her as a Dirge Bard, but using the Tranquil Whisperer archetype you could snatch a couple more points of AC (I prefer Inspire Courage, but once you reach level 8 you can use Inspire Tranquility when you need better defense and Dirge of Doom when you need better offence, even though I guess that the AC bonus of Inspire Tranquility can't compete with the general usefulness of Dirge of Doom... I guess it depends on how many enemies are immune to it).
Now that I think about it, I can't wait to try an all-performers party, consisting only of halfling Bards and Skalds. With the insane amount of different archetypes (Dirge Bard, standard Skald, Tranquil Whisperer, Beast Tamer, Archeologist, Court Poet, Hunt Caller, and Battle Singer) it sounds more than possible.
