Personal Combat:

This achievement belongs to those who've scored 30 or more kills with their SCV in Nexus Wars

You! Shall! Not! Pass! :

This achievement belongs to those who've prevented any enemies from harming their Nexus through their side (up or down) for 3 games in a row in Nexus Wars.

(note, you choose to defend either up or down at the start of the game, if you choose up and your teammates on the down side fall, it doesn't count against you for purposes of getting this achievement).

Never Surrender:

This achievement belongs only to those who's entire team has ditched in the first minute of Nexus Wars gameplay, through no fault of their own (they didn't ask the teammates to quit, they didn't insult or otherwise antagonize them) and chose to fight on defending their Nexus to the bitter end.

What is Nexus Wars?