No markings usually means "target the monster the tank is targeting". I only use numbers when people are too stupid to figure that out and every single-target dps attacks something different
Now I don't know how well your aoe is developed in that dungeon (depends on if you hit 30 already or not), but since the aoe strength of ARC/SMN below that level is not all that great, it's not wrong to stick to single-target dps as well.

Oh, and the actual fault lies with the healer. Nothing in Haukke normal should be able to wipe a party anyway.