That's too many premium units, but glad it worked for you. The following stages are ridiculously easy in comparison, even in hard mode. If you can clear 18-3 you can do everything else. Nice roadblock there, Lilith.
Oh btw, I just cleared 18-3 on hard mode on my sub account last week. Was an ass of a struggle to keep everyone alive since they're not as geared/awakened/petted as my main, but I eventually found a solution - Valan, Zem, Nighshade, Mazir/Alecia, Baltor. Nightshade's strat is to eat the initial bombing, which puts her near dead and fills her ult gauge to full. She then dashes to the orther side and baits the nuker and two bombers, none of which can hit her because of their wide shooting arcs. Once those are dead, use ultimate to help kill the magic-immune golems. The real challenge is wave 3, where you need to time your mage's ultimate to interrupt the goblin bombers' takeoff to avoid an instant wipe, and then kill everyone fast enough to not let the goblin bomb explode. Took me a good 20 tries to make it work with no deaths.