1 - Trolls, ugh, nasty brutes they can be, especially at Level 7 (where I am now). Well a rather frustrating thing about Trolls is that they slowly regenerate their Health, making them hard to kill at lower levels when the best you've got is probably a Silver Battle Axe. Well I learned quick to jump on large rocks where Bandits and tough creatures can't get you. I find it unrealistic that NPCs (characters AND creatures) cannot jump, but oh well. Anyway, while on this rock, as long as you have a long enough weapon, you can get close enough to the Troll (while still on the rock) and hit him without him returning fire so to speak. Level your Blade, Blunt, Restoration, or Destruction Skills by letting their Health come back up, though I don't think Hand To Hand works cause you can't reach.
2 - Necromancers, easy enough to kill... most of the time. Most often, Necromancers will summon a creature to help them fight. Often also, it's some form of a Zombie. Zombies are unique as they're somewhat slow, their attacks are easy to evade, and they seem to step back a bit when attacking allowing to remain in one place while fighting them; plus they have a lot of Health. So Level any of the Skills mentioned above.
Leveling Blade, Blunt or Hand To Hand takes a LONG time around 40 (skill level) so finding something like those two examples is great so you can just keep whacking them with whatever you've got. Hope that helps some.