Welcome to a guide on how to send your rare purple hat to the Nihilanth.
The Rarest Specimen Achievement Guide
Xen and Gonarch's Lair
After sending the rare hat to xen, you should find it near the HEV suit and headcrabs. I recommended speedrunning if you memorised the maps well, but watch out for barnacles (and underwater barnacles too!) along the way. I'd leave the hat somewhere that you can remember after dealing the Gonarch, especially the second fight.
I do quicksaving in case if you happen to lose the hat. Quickload [Default: F7] if you lose it. Try your best to rush all the way until you reached the factory's conveyor belts.
In bm_c4a3c1 (the part where you face a swarm of controllers), leave the hat at the start and gluon blasting all the xen controllers and then turn back to retrieve the hat. Beware of the grunt capsules!
The next map, leave the hat near the shower, but be careful when the force-field bubble catches you.
Finally, you elevate your way up and prepare for a huge wave of controllers (and grunts).
At the start, place a trip mine next to the vent.
Then, put a grenade or a satchel on the next vent.
This one is tricky, what I did is circle around the platform to evade the tau balls. Be careful on the first battle as your hat can get knocked off from the arena.
The second battle is a little easier, I just stuff the hat in one of the crystal shield generators.
The controllers will then stop you from reaching the Nihilanth, perhaps the most gruesome sequence. Place the hat close to the centre and go trigger happy as long as the hat is on the platform.
The Last... I am... the last...
Congratulations, you have limitless potential!