I figured this out on my last attempt fighting Moder yesterday when I was about to test out a theory I had after watched some Moder fights on Youtube.

This was also my third time fighting this boss.
The first time was single played in my first own world and the second time at one of my friends world, and just being 2 players was definetly harder than single play.

Anyway I prepared myself with the best food I could muster and had fully upgraded silver armour, sword and shield.

Went up to the mountain early morning fully rested and surveyed surroundings and killed of any nearby drakes and wolfs.

Activated Bonemass and let the show began.
This was it and oboy I was nervous since Moders flight attack is not to be played with and yet I was about to challenge that.

I was about to stand dead still on the spot with my shield up faceing Moder.

The first attack came and she missed me completly, second attack and again she missed me completly, third and the last flight attack and one of her ice block actually touched me but I got very little damage. WTF I thought is it really this easy.

I went over to where she landed and melee her with both first and secondary attack and took more damage by here ice breath than the flight attack.

This was repeated through the whole fight 4-5 times with exactly same results and I did not waste a single arrow.

Try it out if you are curious and see if you get the same results.
I do suggest to choose a Moder place that has more flat surrounding area if possible.