Humbaba also has divinity. So lol chain applies to him as well. In fact Enkidu made the killing blow to Humbaba which is why he was cursed to die fighting Gulgalanna.
Ishtar must be really dumb. Why would be plot a careful revenge using monsters that the heroes already know how to defeat? Gilgamesh is pretty much in his best form and he even has his wingman. Ishtar better have something much better than just the same old monsters