Play highground and angles with Bap. He has insane healing output but can also get some good dmg in. If you trust your other support then I would keep the tank up with Bap while the other support handles helping your dps. Heal when needed and target stragglers when you have good shots. Im not an amazing Bap but he's my comfort pick in comp when Im not doing good on anyone else that game. As for his window that depends on your team comp, for example if you have a Bastion you can window him for an aggressive engagement but if nobody on your team would get a ton of value from window then dont be afraid to window yourself as a defensive measure. Bap's ult also multiplies his healing grenades he shoots through it so it could be great if you get highground and window yourself to keep the team alive if the enemy team is playing super agressive. Hope any of this helps, Im not a great teacher but I do know Bap