I've noticed a strong correlation between buying witches/mages and winning the game. So long as it's not a stomp:
If I buy lots of mages, I win.
If I don't buy mages, I lose.
Part of the problem is that druids (the strongest unit in a spawn wave) are critically weak against nuke spells, and upgrading to mages is currently the only way to deal with this weakness. It doesn't matter how well I cover the spawns in a push. More than half the heroes in the game can get away swooping in to kill a pack of druids just before they start attacking towers.
Mages deal with this problem by having enough hitpoints that it takes more than one Wake or Leave it to Cleaver to kill them off.
Unfortunately, they overcompensate by also being the best anti-hero spawn, the best anti-spawn spawn, and the second-best spawn at piling up in a siege.
Add alternative ways to keep a spawn wave semi-safe from nukers. You want to swoop in and murder an entire wave with an AOE nuke? It should have to be a 1-2 punch.
The 1 in this 1-2 punch can be a variety of things.
- Picture an antimage unit with an aura that significantly reduces spell damage. He can have otherwise mediocre combat stats; the important thing is that, if anyone tries to crush a wave without picking off this dude first, they'll find magic only somewhat successful.
- Picture an antimage with an autocast buff that significantly reduces incoming magic damage. Balance the cooldown and duration so a single antimage can protect perhaps 3-5 of his fellow spawns. This way, there's an incentive for the attackers to keep at least a certain number of these antimages alive; if the attackers balance their upgrades so they aren't blowing money on redundant antimages, then the defenders have an incentive to pick off the antimages of an advancing wave.
There are, of course, countless other things that could be added to make it trickier to wipe out a wave of spawns. In any event, though, that a defending hero has to pick off one specific unit before he can instantly massacre the rest of the wave.
"Oh, you have promise. But wait until you have more years fall upon you, and you will see what a shell your heart will become."