During the last podcast Vinny and Ryan mentioned some points about this game that, as I found out playing after listening to that bombcast, turned out to be wrong:

1.  Going to a completed campaign:  Funny thing, I tried pressing the d-pad in the map screen and you and highlight the storyline you're currently or one you've already finished.  This gives you the chance to finish side quests and bounties for those Achievements.

2. Grinding:  There are caves in the first and second campaigns (that's as far as I've gotten) caves with a crossed swords sign in front of them.  If you move from point to point you'll eventually be ambushed by a level 5 enemy.  It only gives you about 250 xp so it will take a lot encounters to max out all of your units.

Hopefully this helps anyone worried about missing Achievements/Trophies.