That mage is meant to either be killed quickly with a longbow or avoided, I can't think of a way to get him to switch weapons since disarm doesn't appear until Radiant Dawn.
As for Tibarn, yeah, generals and paladins can rescue the birds in either form to keep them from stealing your precious exp. Especially helpful if you want to get the adept scroll from the guy near the boss. It is worth noting, however, the birds will not attack the boss (or at least they never have for me) so if all else fails, you still get boss exp.
Regarding completion time, you seem to have figured it out, but this level has very few reinforcements, and since you just have to kill the boss to win, it's a lot easier to complete quickly than a rout or defend for 10 turns chapter.