One you've killed Plant 42 and obtained the Helmet key, go the second floor (actually third floor) of the mansion where you got the Armour key. Go to the end of the hall and use the key.
Some tips for facing Yawn:
- Acid Shells do the most damage of any weapon you'll likely have (if hit DIRECTLY). One shell depletes about 1/3 his health, use them if you have them.
- Yawn is predictable, his attacks are easy to read, as long as you fire and move, there should be little chance of taking damage. A good idea is to fire, then run around the bookcase, waiting for him to follow. Repeat.
- Blue herbs wont cure his poison, but still bring green herbs or First Aid Spray to tide you over until you obtain serum if bitten.
You may want to save before fighting Yawn, just in case the achievement doesn't unlock. Per MXR in the comments.
If you're going to down vote, at least let me know why so I can fix the solution.