Alex Sobecki's guide isn't wrong; it just doesn't provide enough information, imo. Special thanks to the commenters on his solution as well.
The achievement can be done solo.
From the main menu, select 'MULTIPLAYER' > 'ESCAPE'.
Move right to 'CUSTOM', move right to 'HOST PRIVATE' and press
to create a private lobby.
Change the map to 'VENOM RUN' (currently the final map).
You might as well select the 'BEGINNER' difficulty as well.
Press
to start the match.
Equip your rebreather and get swallowed!
Run forward through the map until the biohazard countdown hits zero and the venom is triggered.
Now keep an eye on your minimap.Stay within the light green area without letting the darker green encroach on you (as this harms you).
The first 5 enemies are D.B.s and can all be executed by walking behind them and pressing
.