it's easy if you will get 2 items (one more is optional) and 1 spell and you can just go to the boss after it.
1 - get dragon wand for better damage and one spell
2 - get Pyromancer's Mantle for even more damage and same spell
2.5 (optional) - get Aegis Amulet for deffence for battle to be easier cuz you can drop the card for mana and 1 armor and not just 1 mana
3 - get spell Incendio (get to building which clones your rare spell and clone it and you can use same building to clone incendio before boss battle)
4 - use protego minor in a deck 4-5 times and Incendio 4-3 times
5 - at the start do 2 Incendio to the snowman and maximum your defence to get as little damage as possible (with aegis amulet and a bit of luck you'll get 2 incendio and protego maxima at your first hand so you'll use 2 incendio drop 1 card and use protego maxima to get enough defence)
6- use incendio as much as posible without taking too much damage