I understand the grind and a milestone you don't like getting to you, but my experience is that Vanguard is extremely powerful (up close). I get killed by this all the time. I've done the challenge and admittedly I"m not all that great with Vanguard, but I was able to complete it within a fairly short amount of time. I personally don't really like the Assault class and mainly because I too don't like using Vanguard, but I assure you, as you play this game you will be killed by it all too often.
Each class has it's signature ability that is on a timer and can only be modified by cards, not swapped out. So I don't see that being an issue because it's just like the others.
I'm a completionist, like you, and I will tell you there will be milestones that are infuriating. Just wait until you try for Phasma's final staff strike. I can't tell you how many games I tried going for that one and dying miserably because it's a lousy ability in its current state.
Anyway I wish you good luck. I'm sure you'll get it soon enough if you stick it out.
