I think this achiev has a misleading title/description and it's actually much easier to get in solo mode (but obviously still not as easy as gettin' it with a boosting partner).
You can earn this achievement not only by getting eaten by THE monster, but by wildlife aswell. I got this achievement 2 times (on main and dummy account) being eaten by a Reaver (
http://evolve.wikia.com/wiki/Reaver ) and not by the Monster itself.
Pack of Reavers:
You will probably need some tries to get the achiev being eaten by the Reavers, since their spawn in packs is not guaranteed.
Without further ado:
1. Select "single player › custom".
2. Select Arena mode and an underground map, since rocky environments seems to slightly rise the chance for Reavers to spawn. I personally got this (twice) in the Broken Hill Mine.
3. The important settings you should specify in the "Extra" section, before starting the match, are:
Difficulty: Balanced
Wildlife: High
Strikes: 0 (we want our hunter to STAY dead)
Battle Wins: 3
Battle Duration: Infinite
Monster Armour: 100%
Monster Stage: 3
Sudden death: no
4.
DO NOT play as "medic". The AI chases the medic as first target and you will not be able to stay away from the hunter bots.
5.
Select Goliath or Kraken as monster, the DPS of the other ones may kill the hunter bots too quickly.
6. As soon as the match starts, DO NOT follow the hunters, but search for a pack of Reavers far away from the fighting monster and hunters instead.
It's important to stay away from the fighting area, since the stage 3 monster will otherwise scare all surrounding wildlife.7.
Important! Reavers will attack you only if in packs: an isolated Reaver will run away from you.
8. Let yourself be killed by Reavers. You have a 50% chance that a Reaver will start eating your body after your death.
The reason for starting with a stage 3 monster with 100% armor is that it will give you more time to look for Reavers and the monster itself wont try to eat any wildlife at the beginning of the match.
Note: if you want to try the Reavers method AND a chance to get actually eaten by the monster, start a match with
Monster Armour: 50%
Monster Stage: 2
instead of
Monster Armour: 100%
Monster Stage: 3