Ok, for those who are interested in what the addon is all about, I have just played Tier 1, and would like to make things clear. Is it fun? Yes.
You have two tasks before the main boss, each task is two levels each. The first is to destroy the artillery platforms (there and back), then you have to open/clear the gatehouse (there and back), then you have the final level against the boss, more on that later.

Firstly, your enemies. Mostly gremlins, occasional gun puppies and rocket puppies. However the gremlins in question are joined by two new types. Firstly, ones which carry rockets around and are shielded from the front. I reccomend playing with a mate to stop this, as they always back themselves against a wall to stop being flanked. Brutal and over-zealous use of the Blackhawk also worked. Every now and then they will stop and fire a rocket into the air, and a bomb marker, like those when fighting the Twins, will appear on the ground.

Second new enemy was the invisible son of a ***** gremlin. Using saw blades and wearing a red cloak, these assassin gremlins sneak around generally taking you by surprise. Right at the start, they gave me a rough location by having the same effect as Recon in PVP, a skull overhead letting me knnow they're in a circle region around me, and once you were marked, they would appear and attempt to **** my **** up. Never the less, my Acheron proved to stop this fairly quickly in tier 1.

There are also new monster spawners that look like an old stove with barbed wire around them, fire through the open door with a projectile weapon once or twice will generally break it. The other things are more rockets (ala munitions workshop rockets, so prepare for fire and a good shield), and also invisible mines. These will only appear if you're near them, and my humble advice is just keep running, because they like to spawn in groups, meaning some hot-foot pace.

Also, right at the start, mission lobby, there is a crafting table, where you can use your fractured pieces of boss mask to make masks for yourself. I got one from my last round,although I did die once trying to beat said boss.
As for the weapons you recieve, this is where things get interesting. You are given a rocket hammer and a bomb called "Dark Reprisal." The bomb causes a small explosion and causes two shadow lights to circle around it, and what ever those lights hit, they do damage. Pictures at bottom. The hammer is about the same pace as a troika, but it's charge does a slam on the ground, step, and then slam on the ground again. In ATH, this charge attack did 38 per hit on the luckless spawnable droids. The Dark reprisal orbs did 8 per hit, but because they move around quickly in said circle, I did about 24 to one. Please note these are only two star weapons. Pic links underneath.

http://cloud.steampowered.com/ugc/470863048306103334/B4ECF2D497E59EDB0DF... Wait, what's that?
http://cloud.steampowered.com/ugc/470863048306101081/73F84EF9E9CE20F4117... The invisible one reveals himself
http://cloud.steampowered.com/ugc/470863048306097018/93F8DAF5C328CAA7311... The pitched battle
http://cloud.steampowered.com/ugc/470863048306099726/58F2DA709DCE80221FD... Foreboding entrance to the boss level
http://cloud.steampowered.com/ugc/470863048306093943/19730A915792B6D12C6... The arena itself
http://cloud.steampowered.com/ugc/470863048306098425/9161A0183B56AADBD03... Dark Reprisal, the 2 star bomb