I would assume you're speaking of Behemoth (even though Najort is after Trojan [but he still only gets spell immunity at about 25% HP or something]). Having high level walls, high wall armour, good repair units and bots for your walls gives you sustain to be able to kill him without him getting through. For damage output, having a hero with a few cursed larcenies (which are 4 black larcenies combind) can put out a lot of damage quickly; if you're using towers, having assassin, flame, rock, gravity, and maybe ice [don't think it affects him but it's useful for other enemies] will also put out decent damage and you should be able to kill him, provided you have enough damage and sustain.
Also, for a tip, when he starts to fly up a bit to shoot his rockets/headshot death attack thing, he loses his evasion temporarily.
Lastly, it would be nice thing to know what difficulties you've tried doing this on, so it could be refined a bit.