I played the above list against a Clan Skryre list and it was kind of silly how much damage Kroak can put out now with some setup.
Astrolith plants flag
Kroak uses command ability to generate insights
Starseer attempts to generate insights
Slann adjusted the constellation for +1 to cast
Kroak summons a Balewind Vortex
Starpriest summons Spell Portal
Starseer summons Cogs, Slann touches Cogs
Kroak launches Celestial Deliverance x 3 with a ton of bonus range (+8" from flag, +6" from vortex, +6" casting result 10+) through the portal (ridiculous in the realm of Shadows) + a Comet's Call
Slann generates 12 Conjuration Points (+1 for Slann General, +1d3 for Astrolith) or casts spells as needed
In my first hero phase Kroak alone took out:
2x units of Acolytes
2x Warp Lightning Cannon (and a third cannon left at 1 wound)
Arch-Warlock
Warlock
10x Clan rats from a 40 block
3x Mortar Teams
I also dropped 20 Skinks from the Engine of the Gods and called up 10 more Skinks using Conjuration Points at the end of the movement phase. We called the game after that.
The reliability of magic with the buffs we can stack is great. With Astrolith, constellation, and Arcane terrain you get +3 to cast which makes getting the bonus range from 10+ pretty easy to get. Having insights coming from Kroak's command ability (which can then refund CP on 5+ from the Aetherquartz Brooch on the Slann and then do it again and again) and the Starseer is great. The insights really make casting reliable as well as making it easier to get the range or wounds needed from Celestial Deliverance or number of targets from Comet's Call.
Engine of the Gods near a Slann is rolling 4d6 with potential insights for rerolls as well as Curse of Fates to modify a die. Not guaranteed to get what you want, but pretty damn reliable.
From turn 2 onwards Kroak is casting 5 spells and the Slann is conjuring 4 as long as he keeps touching the cogs. If you need more offensive spells, let Kroak touch the cogs and he's on 6 spells from a Vortex through a Portal. Geminids and Palisade are in the list for debuffing and board control - Palisade is also just great for hiding your magical frog fortress.
The only army I see really being an issue is Fyreslayers maybe because they're quick and can have a very strong save vs. mortal wounds. However, if the characters die they lose their buffs and Celestial Deliverance + Comet's Call are great for removing every hero at once. They kill the first Skink wall, you make another and keep blasting. Also, Spellmirror and Lens of Refraction would be fairly strong as well but again just aim to snipe the characters carrying the problematic items (use those insights!).