Actually, it is very easy to come back from a defeat on Cruciible. With just a little intelligent play, if the enemy bought giants you can farm up to level 20 and artifacts, lock your portal, buy giants, and win. As long as you keep your portal constantly locked, your health crystal is so close to your last tower block that you can easily spam AoE moves on cooldown, never die, and generally distract the giants too much for them to actually hit you. Basically... you simply decimate the enemy forces, get massive EXP and gold, and then quickly buy up to giants yourself, go buy artifacts, and push the enemy; if you didn't feed them much, your gold will allow you to push better than them, but even then it could just become an infinite standoff.
If your enemy is smart.... they will just constantly gear up and charge your towers, slowly killing them... and then unleash giants when you have no more defenses (if then!)