The Achievement itself is to get 10 Bloats inside the bile without letting anything else fall inside.

The Bile part is the 4th Phase the other Phases being:
holding them off (1st)
searching for the Crane tools (2nd)
sitting on the pressure plate (3rd)
Bile tanks (4th) <--- Your Stop
Getting Gas (5th)
Surviving the Horde (6th)
Patriarch (7th)

To make this as easy as possible you'll want to do this on beginner since zeds usually die in one hit to almost everything.

Through the first 3 phases you can choose just about any class Firebug being the easiest with the pressure plate part or zerker. support can be used on the 2nd phase to open doors more quickly.

For the 4th Phase you'll want to go commando and pick up the SCAR or FAL since at lvl 6 commando, they one shot clots and just about everything else, FP's die to not even a full clip of the SCAR or the FAL, but if you somehow don't have enough money, The AK will do well, you just have to shoot a few more times.

When you do finally reach the 4th Phase you'll have to do these
-Go Commando
-Buy FAL or SCAR
-Take out anything that isn't a Bloat

Now there is a special spot that will make all ZEDS move towards you while bloats will jump in the Pit instead as you can see here in this screenshot below

As you can see there are pipes across from you, perch yourself against the middle of the pipes on your side.
(You'll be on the correct side if you look left to see the last trader area)

Again all Bloats will move towards you but then change their minds and move towards the pit, just be careful not to blow them up as this section can take up to 10 mins.

This achievement takes more time than it needs to. but just having one thing jump into the pit that isn't a bloat will not give you the achievement.

Good luck achievement hunters.

Notes: If you do succeed in getting only 10 Bloats, the Achievement will not be given until after the wave is over.
Husks can move you out of your spot so back up against the pipes again if he manages to hit you
Make sure you're loaded at all times
Always check both sides frequently.