Kotl is extremely fragile and slow and weak, the best way to tackle him is to get tanky heroes that can bully him out of lane or outright kill him like Nightstalker, Sven, Centaur etc. supported with Jakiro, Veno, Shadow Demon, the likes. If you let him just pummel you and your creep wave from afar, you'll have a lot of issues. Kill him early, kill him often, and he'll leave you alone. You figure it's OK to tank an illuminate while walking up to him if it means you'll murder his bony ass.
And if he's just casting the nuke in fog/forest, keep some wards up too, sometimes the audio cues just aren't enough.