I confirm, it was a case of counterstrike vs tank. Usually Siam use another strategy against wars to avoid that but I had deactivated it temporarilly while I was doing a nightmare run so as to make sure he stays on his default strat of counterstriking whatever the mix of opponents.
From my experience, Counterstrike vs counterstrike can produce that effect too...
Anyway, it was worth it, look what the little cat dragged in from his run :
Chain Mail Arm Guard of Rru-Uyac
(Soulbound)
Medium Armour Weight 6.1
Arm
157 Armour
Durability 60/60
Item Level 50
Specialties: War, Theatrics
+ 14 Strength
+ 16 Intellect
+ 9 Agility
+ 10 Defense
+ 82 Health