Yes, Nether strength is related to your character level - so outright hammering one into submission is usually not going to work. Here’s the thing, man: you’re not supposed to just be able to flail one to death. You have to leverage things that don’t rely on getting past those enormous stats. Here’s a couple key things to have, and then the main routes to killing Nethers.
Raven Acolyte: 50% of a Nether’s health is a hell of a lot more than 50% of your health. Enough said.
Delaying Effects: Time is always going to be your shortest resource in a Nether battle. If you’re a Life mage, you can stack Holy Defiance and basically guarantee that your creatures are going to take at least three hits (and at max rank, four). If not, you’re going to want Stuns and action order effects, or something to make yourself not die. Spells are best; Petrify, Stasis Sword, Mind Tricks, etc.
Ressurection Spells: No joke. Have one.
Lose Your Conservatism: Don’t try to conserve shit, this ain’t the pick-up games. Cast spells and a lot of them. Use consumables before every sigil battle, and not the “more resources” crap. Prepare.
One way is to take one of your creatures that isn’t your main DPS, and give it a weapon with a straight list of status effects, though this is slightly unreliable. Far better is reliable methods of delivering status, which involves certain creatures listed below. Another way is to use attacks that don’t rely on stats or deal straight percentage damage, though those are rare, and Arsonist is by far the best anyways.
Lich Shadowcaster: Guaranteed Bleed and Silence on hit. Bleed is 30% of Speed, so Nethers bleed out like they just had heart surgery.
Necrotic Clutcher: Guaranteed Stone, a 25% chance to kill per hit after that. Kind of chancy, but lets your other creatures help.
Noxious Smog: Lots of Poison - at ten stacks, it’s 30% damage a turn.
Death Guard: Doom - a three turn countdown to victory. If you use this and Raven, then deal at least 10% damage, it suddenly gets a half and half shot at just killing the Nether instantly.
Troll Arsonist: 35% damage a turn free. If the Nether’s the only thing alive, it won’t survive much of this, and Raven Acolyte makes this almost free. In fact, a team of four Arsonists, a Siren Coercer, and a Raven Acolyte can pretty much freestyle on Minor sigils without wearing any artifacts at all.
Phase Marauder: Has a fair chance to ignore Defense when attacking. Useful, but not as much as the rest of the list.