Hellmouths

You definitely need a plan for hellmouths. If you dont really have one, you will probably want to flip the table. That said, they are not terrible if you have a plan. My last game vs hellmouths was with Menoth. So I just had the idrians Prey one and walk up to shoot it. Done.

To that end, terrain providing, I think higher POW or boostable shooting are the best answers. If youre in an all-melee list, things can get alot harder. Or if the Legion player can exploit terrain to hide the maw effectively. A pureblood is not a bad take. Boosting damage on the spray will do about half the maws damage on average. Then its fairly easy to pick off with, say, a WRC or LotF on average dice. Alternatively, bloodtrackers with Prey can advance and throw, or possibly charge tentacles and quickwork shots in. Depending on placement, Ambushing bloodweavers can also get at the maw, or force tentacles away from the fight.

The hardest part I find is player skill and terrain, though. A skillfull player with favorable terrain can put a maw in a tough spot, and you just need to plan around it in some cases.

Also remember that he can only put one tentacle into play each turn. You can control what the hellmouth can do on your approach by gunning down as many of the tentacles as possible and then body blocking the pulls with shifting stones or other hard to remove models.

Just don't give anything away for free.

Thats the annoying thing. I cleared off the tentacles super easy and nithing got pulled, but it still messed up what could have been a scenario win. It felt like i had really good tools to handle the hellmouth, but it turned out i just couldnt kill the damned thing, and it saved him on scenario big time.

List and placement do matter, of course. The list I faced last had no BB. Body blocking, as suggested, helps if you need to chip it over a couple turns. Putting attacks into tentacles can help too, if you can spare them, as also suggested.
One option is auto-damage effects, or Grievous Wounds effects to allow you to accumulate chip damage. So Alten could be an option, for example.
Failing that, a disposable light might also do.

Put this way, what was your full list? Krueger2 should be able to tag Rebuke into the unit, then trim tentacles too. The rest depends on the list. And of course on table and opponent.

Could you have counter-deployed the Well to shoot it?
Seems like the best use/investment. Maybe finish with a WRC once wounded.

Rebuke is handy in stopping tentacles trying to to do work without dragging.

Soooooo ... now you know what fighting against sentry stones feels like

The well couldnt see it because of a forest, and its still Rolling dice minus 6 on two dice, so it will take a while hehe. We did play the interaction between circular vision and backstab wrong though. We played it as if backstab didnt work, but it should, so then you get some damage in, but its still dice minus 11 on three dice and dice minys 8 on two, so... Not killing it in all likelyhood