Vader cost me my GAC

I was using Vader in my Dr Aphra team and while he was using "Merciless Massacre"  he got counter attacked by a defeated death trooper, and just stopped working. I tried pausing, putting on auto and nothing would snap him out of it. I was facing an Iden lead team with Range trooper cron so that was my only counter to with them in the back wall. Is there anyway to avoid that from happening in the future???