Hello Tarantula , Block revive champions are fairly easy to come by in teh game and canbe particularly powerfull , an example that comes to mind that can be great for that stage is Armiger , he has turn meter control, Max ennemy HP based damage ( which hits hard ) and a block revive, and best of all he's a common. The frost spider is actually one of the easiest of the bosses we've had to face in the tower you can check the video below from hellhades to see him clear it with easy to get champions.
https://www.youtube.com/watch?v=VIja_CJY9aw&t=819s&ab_channel=HellHades
On the point of the key system, it is possible to miss several clears and still finish the tower with a bit of planning, but remember the content was not meant to be cleared easilly so Hopefully it'll motivate you to push your teams and think of new comps. Don't hesistate if you need any further help.