What about upgraded harpies? (fighting them would take much longer since there are far fewer of them, but the damage is almost the same, except hardly anyone builds them on the first week). And what about shooters with an ammo cart against some golems and zombies? (You can kill an infinite number of them, the question is how long it takes). In general, the argument is unconvincing IMHO.
A scrub with 1-2 upgraded fairies goes out and sees many skeletons. And breaks through.
Never seen a scrub with a shooter and an ammo cart. Or with upgraded harpies on the first day (more often they'd upgrade Troglodytes on the first day, after all).

Орден Феникса – харизматический :) лидер