ill break down exactly what happened ingame.
it was my turn and i used Deathfire Crescendo (Give a friendly minion, “Deathwatch: Gains +2/+2”) on a Wraithling. i killed 2 enemy minions and now my Wraithling is at 5/5. With this, i attacked his General, and was now at 5/3. ended my turn.
his turn. he attacked the Wraithling with a 2/2. W is at 7/3. attacked with another 2/2. W is at 9/3. summoned a Mirage Master (Opening Gambit: This minion transforms into a copy of an enemy minion), copied my W at 9/3 but was immediately killed and now my W is a 11/5.
it seemed he was confused about what happened along with myself. we’d thought it would transform into a 9/3 minion.