I’ve used Kwaak and Gub quite a lot, but somehow never seem to get a lot of value out of them. I know they’re a super powerful model, so how do you guys use them to good effect? Which spells do you cast the most with them and why?
Well, you're taking them for enliven so plan around that. They'll get to do work with venom in most games, and occasionally heal your beasts. Mudfoot is exceptionally rare, but is something to keep in mind especially if there's an enemy gargossal (can also combo nicely with dracodile floodwaters). Main knock against them is support bloat and the competition from soulslave/magicfishstick/pathfinder eating solo slots.
Worth noting that enliven isn't great by itself but gets quite valuable when layered on top of other defensive tricks, including -Skarath: high def,steady,counterblast/paralysis -Spitters: carapace takes the oomph out of free strikes, counterblast -Lug: countercharge, hyperaggro, return -WE&SJ: unshootable, and starcrossed on the free strike -Gun Boars: counterblast -Area Denial abilities (Mire clouds, Longchops Trap, vault soulstorm); -Ranged defenses: Enliven's doesn't help vs ranged, so Valkyries for shield guards, mire cloud wall are helpful -Husks: even more during-your-opponents turn chaos -Other defensives buffs: Sturm's vision (negates the free strike damage entirely) or polarity, Helga's Defenders' Ward (especially if it's skarath), Carver's Batten, swarm, maelok feat, etc)
All that is to say I have found them to be an effective component in a Helga list (targeting the Defenders' Warded Skarath and a sprinting Road Hog which tries to get back behind the mire/longchops AOE line), and in Sturm lists (skarath/snapjaw) with the full husk boat causing havoc during the opponent's turn
Honestly, Kwaak and Gub are a model I still have some difficulty with. I've kept them in my Arkadius list ahead of easier-to-use models like Hutchuck because every now and again they win you the game. Other games they'll have close to zero impact though, which is largely down to the fact that Enliven is their principle early-mid game impact and it's an ability that opponents have a large capacity to mitigate. However, Enliven on its own is huge, and Kwaak's remaining kit can have a huge impact late game. The biggest uses I've found are:
-Enliven against enemies with multiple, strong shooting attacks, particularly against lists where the enemy doesn't have many other ranged attacks to trigger Enliven early. Judicators, Vulcans, and Animantarax are what I'm usually playing around with this. The idea is that you set up so that the Enliven move will take you out of the maximum range of their remaining weapons. The Judicator is the perfect example, especially since it's often the only major shooting threat in a Protectorate army. If you're ~14.1" away from a Judicator, it can walk up (to get to 9.1") and shoot with you its spray or rockets. The first time it does damage, you advance 5" straight backwards. Now you're 14.1" away from it again, and out of range of any of its remaining guns. Even against the Animantarax, which has an eye-watering 23" shooting threat between trampling up and firing its guns, you can dictate its position and ensure it won't be able to make much out of its Reposition move by forcing it to be within 7" of its target if it wants to get all its gun shots off. And most opponents won't be accounting for this on the fly, so you can often take players off guard. I should note that for this to be most effective you basically need to be playing Arkadius (which is where I am playing Kwaak) or Rorsh and Brine, since you then need to be able to threaten something like 15" or so to retaliate in your turn. -The utility of two Magic 7 Venom Sprays in the mid-late game should be obvious, particularly since Kwaak is usually using them on enemies who have committed into melee and clumped up in the process. I've erased most of a 10 man unit with Kwaak alone on multiple occasions, and the Continuous Corrosion component expands out the variety of targets Kwaak can kill. -Mudfoot is hard to use, but when you do you can lock critical models out of the game entirely. This is never going to get used before Turn 3 at the earliest, but I've had some amazing moments with this spell. Almost all against Protectorate, now that I think about it. Cinerators hate this spell, especially if you can hit them with Crippling Grasp too.
Huh, hadn't realised how much they were keyed into the Protectorate match-up until I wrote that out
As an aside, if I didn't need the Mist Speaker to play into a few lists like Grymkin and Cryx, my Sturm and Drang list would totally substitute the Mist Speaker for Kwaak and Gub. Few things make an opponent miserable like a Stealth (Mist Walker, which Mire makes trivially easy to trigger), Enlivened Skarath with Counterblast and Vision, and there's something viscerally satisfying about the growing pile of tokens next to that one model.