How does MQG's Unique prevent TM reduction

I tried Imperial Troopers vs KBeq Jedi, with Poncho Bros. I waited for all Jedi to take a turn, so "start of battle" clause should be out. During Gideon's mass assist, Piett called Death Trooper to assist twice, and Death's 2nd hit (and last of the volley) stunned a jedi, and this seemed to prevent any TM reduction on the jedi, even though the TM loss is supposed to happen first.

Is this working correctly? Am I understanding the mechanics here? I'm frustrated in being unable to reduce MQG's teams' TM, as I don't understand the limit of his conditionals. Thanks for any insight and feedback.