For this achievement you need to be the assault class.
Vanguard is the special ability cast by pressing
at the same time. Activating vanguard gives you a shotgun which stays activated for a certain amount of time.
There are star cards which will help towards this, Killstreak vanguard being the main one to focus on. This star card extends the vanguard duration every time you get a kill. I would recommend crafting this straight away as soon as you have 40 craft parts.
As per the comment section Blast is recommended for this.