I think there's a bit of fault to go around to everyone. Was a speedrun discussed at the start? The tank say anything at all? Did anyone say anything at all beyond "Hi"? If not then the tank made a stupid assumption based on the gear he saw or just blindly speedran because that's his normal. THAT is on him. Now the DPS...they were taking damage. Were they eating avoidable AoEs? Were they pulling hate? If either one of those is true then the DPS carry blame too. On your end I see inexperience. In that situation I can see why you would hit Medica II but the more correct action there was Cure III. Esuna the tank only and Cure I and II through it for yourself and the DPS. Medica/Medica II and Regen on mass pulls like that will ALWAYS get you killed as a healer since you get enmity increases EVERY time the regen tics on Regen and Medica II.