I see you mention undead. Are you trying that Nybeth battle?
Wait a bit, that one's optional. You can still fight it after a few more battles, so don't push for that one immediately. That one's one of the harder battles earlier.
A general rule of thumb for those Necromancer battles: The best defense is a good offense. While eventually Nybeth will run out of enemies to summon, it's not worth waiting. Especially later on, when the enemies start summoning 4-5 at a time and can keep summoning continually. Just go for the leader, ignore everything else.
As for beasts, I assume you mean dragons and the like? Elaborate, please? Early on, until you get a Dragoon, "Beasts" (Octopi, the griffon-like enemies) and Dragons will be extremely difficult to kill. Stick with Archers and Mages on them. Or, better yet, stick with Archers in general. Canopus is a broken beast of a character and is in the top 5 "best" characters in the game.