I'm unable to reproduce this bug. Can you give me more information? I've tried out the exact same position as you, but couldn't attack the Smoke Demon Champion.
1 - Go inside and attack the boss.
2 - Open the gate and fast click somewhere outside ( at least 2 steps away from it so you won't be attacked by his melee )
3 - Attack the boss again ( or keep Auto Retaliate on )
After doing that don't move otherwise you won't be able to attack again ( unless you do the same again )