That healer clearly has a problem if they only use Benefic II a couple of times and didn't use anything else. Honestly makes me suspicious some healers are bots that were just placed in queue to make it seem as if healers are not in need...

Though my question is how you managed to get a pack of enemies and activate Brutal shell 2-3 times. Last I checked, Brutal shell healing didn't proc on Demon Slice & Demon Slaughter. Were you not using AoEs either?

Best thing to do in that scenario is not pop more than 1 extra mitigation when using Nebula since it's going to mess up the healer's heal plan anyway for the later pulls and assume healer has everything under control, and just continue to AoE. If you die and you get booted out after doing everything right, just report. No need to stress out and adjust too much and waste all your cds immediately, or it could look like you're also griefing the DPS who does AoE and ends up getting aggro as well. Perhaps this is why the DPS and the healer booted you out.