I sometimes do a weak attack with my main team if the raid boss is at full health and very high level (70+). I figure since I only end up damaging it about ~10% and we have to kill it sooner or later, it's fine. I never attack with my main team otherwise. Should I stop doing even that and always attack with a single 2*?

Of course, I always go with my main team and try to kill the boss if the timer is running out.