After the First Elder used his skill Gloom Echo and thereafter knocked out 10 of the possessed Dark elves, Valtar commanded the system silently to remind him, when he is more exhausted and at the right moment when the knocked-out elves could perfectly ambush the First Elder.
While in the normal state of mind someone as intelligent as the First Elder could easily be wary of this kind of tactic, after all, anything is possible since they are under the control of the enemy. However, facing so many opponents at the same time for a longer period had dulled all his senses and tactical mind whether he liked it or not. So, even if this is a typical tactic that can be expected from your enemy, it would still work.
That is why people say, "An army of ants can kill an Elephant."
