Kraken’s definitely THE strongest monster for Defend, but it’s not an impossible situation. The times I’ve won against Kraken is usually the result in either my group doming when he’s about to run out of armor + bringing the fight to Kraken or securing a choke point right after the generator and just not letting him or the minions through. The biggest thing I’ve noticed that makes a huge difference is whether or not the trapper domes both the monster player and the minions away from the relay. The less time they get to hit that generator, the closer you’ll be to victory.
As for the viability of other monsters? Goliath is DEFINITELY a good monster. He may not be able to pull out maximum cheese like the Kraken, but he can sure as hell burst down those turrets. As for Wraith? It’s definitely the ultimate challenge. The few times I made progress with Wraith is by picking off one hunter at a time and then going in with warp blast/abduct to damage the turrets. When I win a Defend round with Wraith, I’ll get back to you on it. I’ll get it sooner or later… !