Saving Kroshkkur from destruction isn't necessary for beating a game, but additional stores never killed nobody. Browse The Crawler's shop before teleporting inside the Maggot - if it have an elm wand of conjuration in stock, buy it (they're cheap). Not only it will be very useful inside the Worm, but you'll probably be relating heavily on wands in future, because in compare of your own abilities they're at least somewhat stable.
For a melee fighters, Writhing Ones are somewhat fragile. You cannot use a shield or a two-handed weapon due both of them blocking your tentacle arm, which means you cannot block enemy attacks or crush them swiftly, and you can't spend a lot of points on Contitution because of Magic training necesarity. Insanity is treated by Zigur as arcane resource, so Antimagic is not an option for you. Your Horrific Body offers you protection and healing, but could simply be worn off in a prolonged battles, rendering you defenceless. So you'll always need the best armour you can afford and a good shielding rune or maybe even two runes - it's not so drastic because you have only two categories need to be unlocked. Learning a Light from Anorithil escortee is a splendid solution too, if possible.
A best weapon for you is one-handed with "of projection" attribute. It launches secondary ranged attacks not only for itself, but for you tentacle arm too. Though that punches are not as hard as your primary, they're making your Insanity soar in battle and can be delivered even when you're Constricting someone.
Ranged combat is your weak spot and always will be. Use Carrion Feet to quickly cover gaps between you and hostile shooters/mages. That skill is going really well with Infravision or wands of clairvoyance, letting you jump to places far out of your regular sight. You can even jump through the enemies (but not through the walls). A secondary debuff placed by it on target foes is short but nasty and almost insures you'll survive the first clinch with them. Overgrowth will prevent you from knockbacking and let you deal a really hard punches. Constrict works especially good on shooters, making their Disengage useless. It still will not block enemy mages' Phasing or Teleporting abilities.
Controlled Horrors is really lots of fun. Though your summons can't move, they pack a good punch and have some really nasty abilities. Decayed Devourers and Horrific Display is good for distraction, while Decayed Bloated Horror works splendidly as a sniper. Horrific Display works even on most of bosses, making some usually harsh battles (like with Ukruk the Fierce or Golbug the Destroyer) much easier. And funnier.
Disfigured Face at first looks pretty useless, but actually isn't. Investing in that skills increases their range (not drastically, but still), allowing you to harass your enemies from a distance with a diseases, fears and other annoying debuffs. Keep in mind, though, that you're still not a ranged fighter and need to quickly engage in melee after casting.
Beyond Sanity skills, while working weird, is still useful in a many ways. Though Anarchic Walk's accuracy is hazardous (and limited to your line of sight), it can be used if you need to dodge AoE effects, egress an unrequired pulling or even try to engage enemy if Carrion Feet is on cooldown. Disjointed Mind is good for confusing or even brainlocking foes, leaving them at your mercy. Controlled Chaos is your dear prize, the only way to cut the game low (not completely stopping it, though) from messing with your damage and cooldowns in a bad way. Note that you cannot acquire Chaos Orbs as you like, they appear on chaotic effects that may or may not be triggered in a battle. It's random, dude.
Friend of the Worm is probably the most valuable branch. Your familiar is not as tanky as Alchemist's golem, but still can take some hits, distracting enemies from mauling you. The worm can't be inscripted but can be infused, so infuse it with a best stuff you have. With its Plague skills, it, unlike you, can actually be a ranged combatant, though without your direct control will be still using Blindside at any opportunity to rush into melee. Keep its leash as short as it's necessary, preserving your Shared Insanity and Terrible Sight. The last skill works on a worldmap too, making a lot easier to avoid persistent Orc or Ziguranth patrols.
Slow Death becomes useful in a lategame, buying you a chance to survive most harsh battles. Digest's time of action is really long compared to equal skills/infusions, so you don't need Heroic items and can save the most powerful Heroic infusions for your worm. The other skills in that category is pleasant, but not too powerful to decide a fight's result. Inner Tentacles or Consume Whole, for example, can give you some health, but Digest itself is far more useful in battle and a good Regeneration infusion will always restore you a lot more of health than Consume Whole. So if your tactics isn't especially based on using enemy skills, you probably won't be investing in that category a lot, except for Digest itself. On the other hand, when and where else you'll get an oppontunity to torment someone in your stomach with a tentacles?