This is just a fun little theory-craft of mine. I like to showcase builds at level 12 so here's something I threw together for a support build:
Swashbuckler (Inspired Blade) 1/Order of the Dragon Cavalier (Emissary) (Honor Guard) 5/Duelist 6
Check out the build here
Anyways, the idea is to use the Bodyguard feat, the parry ability, Opportune Parry and Riposte, and Mounted Combat to divert attacks and soak them up. I'll also have Swift Aid to help boost people around me. I know the character in itself can't dish out that much damage, which is why the Triceratops is there.
My aid another bonus for AC thanks to Order of the Dragon and Honor Guard is +4.
Probably will be constantly moving around the battlefield to find the thick of combat or try to save the squishies.
At the very least I find it entertaining to think of a Duelist doing parries on top of a triceratops.
You can likely find room for a one level dip into the Mammoth Rider prestige class. That will make your Triceratops one size category larger, which is great.