I like the moder fight a lot. The fact it's a mobile boss made static defenses a sort of deathtrap. The first time I tried sniping it from right in front of a wood house... it sort of destroyed the entire house (and me).

After that I build two wood shacks, up on rock outcroppings, to contain portals. I sheared the sides down so regular mobs couldn't get up. I spend 99% of the time as far away from the shacks as possible so they didn't take damage.

Before I summoned moder, I'd sculpted sort of "pylons" out of rock, all over the place in between the two shacks. I used them to block line of sight, so the iceballs hit the pylons instead of me. It made the fight really fluid, I had to constantly keep moving to keep pylons between me and moder. It was a lot of fun (to me).

I summoned moder repeatedly, so all that prep got re-used. I was sort of taking a page from terraria, where you build boss fighting "arenas". Just like the bonemass fight, and yagluth, I did a lot of building/rock shaping.