Just Landed: A Guide for New Arrivals to MP

They raise their arm to hip level and fire three shots with a decent recoil animation. If possessed, they alternate between firing three shots or a volley of grenades. Getting in close will make them melee with a wide, but slow double-swing.

With Melee characters and Vanguards you have to be extra careful with the melee swing he does because it can trigger his sync-kill animation.
He does not need to actually hit you with his melee attack, just performing that attack can make his next move a one-shot sync-kill if any player is within melee range.

The best method to deal with him in that situation is charge in until he performs his melee, then back off until he raises his arm again to throw grenades or shoot, then charge in again.

Fighting two Scions as a Vanguard can be quite dangerous indeed. In that case it might be better to search some cover with right-hand advantage and just shoot one of them. Depends on the situation.