Back 4 Blood

There are four maps in a run of trial of the worm.

You can get bonus points during the mission by completing the optional objective worth about as many points as the map itself.

You don't need to beat the run to unlock the achievement, just score over 100k.

My advice is max out the modifiers for player difficulty and then leave the rest alone. You'll make about 30-40k per map by completing the extra objective and the enemies should be pretty easy because you didn't increase their strength.

Let me know if there is another strategy in the comments and I will update the solution!

Trial of the Worm is an arcade-style game mode where you select modifiers that give a score multiplier and play through 4 random maps.
My friend and I managed to easily hit 100k when finishing the 3rd map with only a 6.9x multiplier!
We kept the difficulty tab on Recruit III (One step before Veteran)
Ridden and Horde tabs we increased to our level of comfort, and we kept the Player tab down all the way (mostly to keep shop prices down).
The most important thing is to not get discouraged when you're picking maps - they tell you how much score you can get from the level (with the modifiers we selected, the maps were saying 15k-20k score for completion). There are collectibles you can find in the map that help your score! They look like little trophies and have an audio cue of laughing (and there's a visual hint in the first map of each run so you can get familiar). Collecting these and completing the objectives landed us over 30k the first match, hit 96k finishing the 2nd map, and finally over 100k finishing the 3rd.
You do NOT have to finish all 4 maps in order to get the achievement. Once you hit over 100k, it'll pop and you can quit out if you wish.