You just need to weaken an enemy so they die in one hit..
It takes 3 kills. You could also having some one pair up with him.
This. Though it can take as little as two, if you are lucky.
Send Donnel + others you may need to the north group, slaughter the Archer the first turn, ignore the Thief for now, and have the rest of your units bring the last unit (I forgot what it was, Barb?) down low enough for Donnel to kill. Since he is at weapon disadvantage, you should toss in someone beside him for the +hit boost. Next turn, if the Thief didn't kill himself, and you can bring him low enough, Donnel can kill him for enough EXP to hit 2.
Pretty easy. Get literally anyone to damage an archer, then let Donnel deal the final blow. In case something goes wrong, the archer can't retaliate anyway.
That last sentence is wrong. If something goes wrong, they can and will attack the next turn. Most likely going after Donny and killing him.
Anyway I can't remember but is there still an archer in the treasure room in hard mode? If so, just lure him into the corner and have Donnel attack him without no worries of him being attacked. In lunatic doing this can get you a level up easily. With Donnel doing more damage and the archer with less hp(don't know how much) I don't know if he still levels up in hard.
I supported him with Kellam and I only attacked with him if there was only one enemy around. I used Chrom, MU, and Stahl's attacks to knock the enemies as close to 1 HP as possible and then went in for the kill. It also helps to surround an archer on all four sides and have Donny attack him repeatedly when he can't hit anything.