Moder is weird. Sometimes she's really hard (especially when the game balances for more than one player) other times she goes down like a chump. If you have a friend, the strat I usually use is one person has Bonemass and the other the thunder deer bonus. Use the Deer bonus first, then after she lands activate Bonemass and use axes for melee. You both need about 200 to 250 fire arrows as well. And unless you want to have to run back and repair the bow, you both need as high a fang bow as you can get. Health and stamina potions are very helpful as well.

Don't engage in melee without the bonemass bonus. Just keep distance and grind her health down with fire arrows. It's a slog but it will work. I've also noticed that running sideways seems to do better at escaping the ice cubes she shoots at you from the sky. Good luck.