Dagannoth Rex is extremely weak to magic attacks, specifically fire spells. Whoever is killing him may want to use the eastern side, as he can be easily fought it from there due to his large size.
Many players trap him as his melee attacks will be unable to attack the player. There are many safespots around the arena, but it's best to use a few if you only want to kill him. The best ones would be on the eastern side, as no kings will interfere unless they have been alerted to your presence (e.g a player moves to your spot with Supreme on him, which makes you also get attacked with ranged)
Probably the worst thing that could go wrong during the first wave is Dagannoth Supreme gets too close when luring Rex. If this is the case, simply go up the ladder if you're close enough, but if you're not, kill Supreme with ranged to avoid having Prime attack you.