Happy Dungeons

While the prologue is the safest method to farm insane runs, there is a much faster way of doing this from pretty much any level, though I would recommend 30+ so you can take a few hits.

To start you're going to want a warrior with the bash ability, gear doesn't matter too much but you could benefit from having defensive, goblin protection, ap/hp up and ap regen runes.
Also turn minions off!

Load up Chapter 2: Infection on Insane, it consists of 5 rooms, 3 of these are optional and the final room you just have to survive, room 2 is the only room where you have to fight anything.

Room 1 - Starting Room
Room 2 - As soon as the room loads go to the left and stand by the barrels, wait as the goblins come closer then go round them to the side and bash them off, continue to bash the stragglers off then wave 2 will spawn - go straight towards where wave 2 spawned and again bash them off, watch out for the spear throwers, a well placed combo will end you, you can knock other goblins into them and to interrupt their attacks and also knock them off, once they're gone wave 3 will spawn - for this wave stand behind the rock where wave 2 spawned, the goblins will slowly come around the corner where you can easily knock them off, you can even bash them through the wall. After they're all dead move onto room 3.
Room 3 - This room is completely optional, since this is insane we will run straight through the enemies to the door, be sure not to get hit by spears!
Room 4 - Run through the maze avoiding any enemies to either the blue or the purple exit, it doesn't matter which. The locations of the doors are the same each time but the layout changes each time. While running through here be especially careful of the spear throwers, they can pop up anywhere and instantly start throwing spears.
Room 5 - Final room, all you have to do is survive for 2 minutes. There is a side-objective for killing 50 enemies here, just ignore it, you do not want to be killing anything. If you're low on health there's a few crates around the room containing food, other than that just stay alive. The robot goblins sprinting around the room are worth about 22k xp if you can knock them off, but it will often get you killed so i recommend just ignoring them, seriously, ignore them! After 2 minutes the enemies will disappear, take the chest and leave the room.

Each Insane run should take about 6 minutes (including load times), repeat this 42 times and the achievement will pop!

It's possible to repeat missions and count towards the achievement.
And so you could then:

go to Prologue (Insane) to this level gives Infinite-spawns. Rinse n Repeat 42 times.

Note: I was Warrior level 20 with a level 50 weapon with 5500Attack.
When i finished the Bosses i had around 32min. to Spare.
So i Think it's doable for 42Missions completing on Insane. That's about 21hours of Grinding.

TIP: if so...Play every day 1 Prologue for 42 days and you won't get frustrated. LOL

Cause there are no 42 different levels to play in the story mode, u need to repeat.
credits to: Dub86Nut

Note: Texans2K2 Pointed out it is possible but not advisable, just get to a high level and
don't wast you're time on prologue because it takes roughly 45 minutes at a time. TY.

28min. with lvl.50 5500attack

Hope this helps !

Each game took me about 3 minutes without much effort and I think it can be useful for many since it can be done alone. If you have a team you will save a lot on spending your arrows.

The method that I occupy to achieve this achievement was:
- Equip yourself (Ice Javelin, Brute Buster, Undead Killer & Thunder Javelin) in "CHARACTER SETTINGS> TOOL BAG".

Now then SELECT "Chapter 1: Quest Knight> Search for the Princess".

In the first scenario, RUN or press to speed up.

In the second scenario many enemies will be released, use 5 TIMES "UNDEAD KILLER" this will end with them quickly since each attempt shoots 3 arrows (Left, front and right).

In the third scenario, RUN or press to speed up.

And finally in the fourth scenario shoot the boss 3 (Ice Javein), 4 (Brute Buster) and 3 (Thunder Javelin).

If you run out of arrows in "Item Shop> Consumable Items" you can get on offer by paying with "Happy Jewels"; (It's random, they do not always put the same items).

I recommend it be a high level or if it is a low level; A team will take you through the mission and it will count for you.

Good luck to all