he was just weaker than all of them together.
try and break a stick in two, and then try the same with a bunch of then close to each other.which was harder ?
same thing, when weak units attack all the same time, their power would be multiplied, so they become equal to a single overpoweer unit, it's a great use of swordmen and specially archers, cause attacking from a few steps away is an advantage, you know, while they come to hit you, you've hit'em already.
anyway, i don't think that daemon giants should be stronger, golems are made of stone, so it's logical for it to be very resistant AND slow, hence daemon giants are made of flesh, so they're logically weaker AND faster.