Well, they need the achiv just to increase success rate in the fight, thats all.
I know those ppl who dont give a chance to pplwho have no achive are tards in real life. How these ppl expect anyone to improve just by reading and watch videos without experiencing the real deal?

Anyway, what u can do is to get geared as much as possible and convince them.

yep.. watch videos on tankspot to get a general picture on how the raid will be going. that will help you after getting full T9/9.5 gear set.

good luck!