First of all when I completed that quest I waited till I was level 33 or 30. Second is that buy alot of healing treasure cards and put all of them in your treasure card deck only healing cards so when nimah or the minion drops your health down to 500 and you don't have a regular healing spell you should get one of those healing cards out and heal. NEVER go under 500 health if you loosen your guard a bit nimah will find a way to prevail. Third is KILL THE GHOST MINION FIRST. One down one to go right? The things I said were the things that I did for the battle. Lucas drakebloom Level 43 Oni slayer Ice wizard
I'll assume that you're an Ice Wizard at around level 28, since they have to face Nimah to get the Ice Guardian spell. I don't know what your secondary spells are.
Nimah is tough for a level 28 Ice wizard to beat. If you have Crowns, the easiest way to beat this boss is to simply hire henchmen. If this dungeon is like others, you can hire three at once. With three level 20 henchmen to help out, you'll make short work of Nimah and the minion for 60 crowns.
If you don't have or don't want to use crowns, then you need to prepare.
Nimah uses level 2 and 3 Myth and Storm attacks, Fire Elf, Ghoul, and Storm Shield. Her minion is Death-based and has 625 health. It has a nasty Skeletal Pirate spell that does about 500 base damage.
I recommend that you go to the Bazaar and buy 4 or 5 Pixie treasure cards (cost about 90 gold each and do 450 healing) and put them in your sidebar, so that you can draw a healing card when you need it. If you want, you can also buy 2-3 toughened Myth shields (about 50 gold each and block 90% damage) as well.
For your deck, load it up with Ice Wyvern, Evil Snowman, Ice Trap, Tower and Volcanic Shields and perhaps a few Snow Serpents. In your sideboard, put the Pixies and the Myth shields.
The minion is your first target. Shield yourself, put an Ice Trap on it, and blast it with Ice Wyvern. This should do about 480 damage, and you can finish it off with a Snow Serpent.
At this point, you deal with Nimah. The general strategy is to keep yourself shielded and healed, use an Ice Trap, then use an Ice Wyvern. If Nimah uses Fire Elf, the Volcanic and Myth shields won't be affected. It will take a while, but Nimah should eventually fall.