but what's a decent medium/heavy armor build?
I think straight Fighter for 20 levels works. Armor Training for dex bonus and get UMD so you can have Nenio craft Mirror Image scrolls for you. Or, grab some INT, go Trickster, take all 3 UMD tricks, and you'll also have a wizard book that you can use to cast personal buffs like Seamantle and Frightful Aspect (end of ch.4 tho) and access to Mirror Image and other buffs from the Trickster spellbook until you get there.
Haven't tried myself, but this guy played straight Shieldbearer until ch.5 on hard/unfair (went Legend on ch.5 with Stalwart Defender and some other classes from what I could see).
Or, Oracle + Battle Mystery + Angel path. Enjoy plate armor, heavy shield and 24h buffs.