Regions Of Ruin

So the most effective way to do this I found is by switching between two areas:

Peaceful Glade (#66) and Tricky Ledger (#191). Killing all on those 2 would net me about 50 rabbits +- 5 and take me 5 minutes (I did it pretty slow so doubling my speed isn't even hard I think).

Numbers are indicated on this interactive map made by theaspect.

Finally I would like to add that you should keep this achievement for last, getting a mage compagnion during your playthrough also helps since when they fire a fireball there is a chance that it pierces through one enemy, hitting all bunnies on the map.

Trickier than it sounds, since you also have to survive the hit.

This requires around 50 Constitution so you can safely escape a 100 Hitpoint Damage.

Use the basic shielding function (The one that blocks 90% dmg), not the 100% block one.

Pump up the difficulty so enemies hit harder.

I did this with a troll which took a couple of hits to give me 100 dmg (through 90% blocking)

Unequip your Physical Resistance gear for faster results.

This can be done quicker with multistrike, a sword skill that lets you hit 3 times in a row very quickly.

If you spam this skill, its like having a deflecting shield infront of you for a while until your stamina runs out.

There is a crazy librarian to the west of the HQ that, if stolen from, throws around 20 fireballs per second towards you, if you can survive, this is a great spot to farm this.

Projectiles from slimes also count for this achievement.

There is a mad druid boss in a forest waaay south of your home base, near the end of the map, he shoots like 20 arrows per second, make sure you have adamantine armor and at least 50 constitution and this will be easy.

Once you unlock the ultimate weapons from your settlement or find a pink rare weapon from one of the storyline endgame chests, this kind of damage doesnt look as crazy.

Using skills like surprise attack that does twelve times the damage and the damage boosts you get to pick will make this achievement easier, its fairly quite simple, just play normally towards the endgame and you will eventually unlock this naturally.

The DLC for this game adds a new gameplay mode called Sieges, you can find the closest siege location southwest from your base. In this mode a lot of enemies start spawning from both sides and you need to defend the location, using the best weapons and armor you have available (or maybe wait until you have your blacksmith fully upgraded for them) go to a siege and tank enough enemies before a powerful blow to kill them in one fell swoop.

Use of followers for this purpose is a smart choice as you can stack up to 25 or more monsters at once, killing 10 of them at once is pretty easy.