To access the platinum bosses first of all you need to be Platinum Donator.
Platinum donator costs $55 to get.
HOW DO I GET THERE?
To get to Platinum bosses, simply type ;;pb it will ask you if you wish to start a private instance or join one.
Once you're there choose the boss you want and enter it's room by passing through the barrier, be sure to pray beforehand as they will attack as soon as you enter.
There are 3 bosses in the PB area; KnightMafia, Demoness & Sigi.
KnightMafia boss is Melee, the best gear/weapon type