The fastest/easiest way I found of doing this is to summon 3 sets of undead using the Raise Dead spell. After that, use a freezer combo like E-W (water wall) combined with E-R-R-R-R (frost wall with long duration freeze in combination with wet enemies). Once they are frozen, just cast a rock projectile either partially or fully charged. Partially charged rock projectiles are usually enough to smash them to bits when they are frozen.

A lot of players also suggest using the Frost Cleaver (ice sword with freeze effect) to do this achievement. It's definitely much easier than casting spells!

You can get Deep Impact by using the same method in my Ice Age video. A few things that help is shooting the projectile point blank. The undead don't have to be frozen so the achievement is actually quite simple.

You'll simply be shooting at sitting ducks (provided they don't have a target nearby).

For this achievement I recommend using Conflagration, Haste/Teleport, Nullify, and Raise Dead.

After the first checkpoint in chapter 9 (right before the necromancer that summons skeletons), you will encounter a large group of undead in a swamp area. You can round them up easily into a cluster by using Haste and Teleport. Once you've rounded them up, summon your own undead using the Raise Dead magick in order to distract the enemy as well as add to the body count for the achievement.

Keep in mind that some of the zombies will be 'WET' so use Nullify to remove their debuff. Once that's done, position yourself and cast Conflagration at the large mass of zombies in front of you for an easy achievement.