Im totally confused.

I went this weird xmas island with my level 15 high elf and i was able to take down this first boss called "Ghost of Christmas Future".

After that came next boss called Ice Hydra and i didnt even bother to try fight it, cus some how this ice hydra deals serious dmg and i will have no chance.

1. Can anyone confirm what dmg type Ice Hydra does??? (is it poison and poison only?)

Also i leveled up new char, level 13 Dragon Mage and i went to fight first boss on island ofc, but my character cant deal any dmg to this boss. (using mind strike)

2. Shouldnt mind strike pierce enemy physical armor????? (why i cant do dmg on this boss)

3. What level players usually go xmas island??? (is it good idea visit island with low level chars?)

Anyone can enlighten me??? pls

The Ice Hydra's melee attacks do Physical and Poison damage.

Its spells and special attacks do Physical/Ice damage. It is extremely resistant to Ice damage, and equally slightly resistant to Magic, Poison and Physical.

It has an extremely exaggerated sensitivity to fire damage. If you have a Dragon Mage try Dragon Strike or Eternal Fire, or a weapon with a lava chunk socketed into it. Dragon Form or Dragon Berserk will also likely make short work of it.

It sounds like you are a very low level to be fighting any bosses really though, especially one who is more like "bonus" content.

I have just started a fresh character out of fun and while I have not planned to go to the bonus challenges yet I've faced the first real boss at level 13 on silver difficulty and as dragon mage. With the berserk form and better life-leeching upgrade I was able to tank and kill it without evading or losing more than 15% HP at once. I can imagine you can do the same against the Ice Hydra. Life leaching is ridiculously overpowered on berserk form even without the community patch, with community patch you're technically immortal and only heavily armored opponents may cause problems or if a horde can spam stun abilities.

I always had the most fun with dragon mage, especially at high level when the dragon form becomes so viable it barelly take any damage, making you feel like a boss lol. Something not even the berserk form can compete with.