I have played SK before about a year ago.
I have just returned and I wanted to purchase the OCH expansion for the 5* hammer.
The question is how do I go about soloing the tier 3 OCH mission since presumably not many own the expansion and I'm about to buy it
In my Arsenal I own:
Shivermist Buster
Gran Faust
Divine Avenger
Voltedge
Polaris
Blitz Needle
Archeron
Vile Striker
Ascended Calibur
BTB
Armor
Full Skolver
Vog Cub body
Shields
owlite 5*
BTS
OK, out of that I'd go with Vog Cub since there is mostly elemental damage dealers (constructs and gremlins) and lots of sources of fire damage.
Owlite for shield, at least on your first run. If you find that you don't get hit often you could switch to BTS for the sword damage bonus, same with switching to a piece of Skolver after practice.
I don't know how many, if any, weapon slots you are using but you have a few combinations of weapons that would work.
Archeron of course, since it's recent damage buff, great for gremlins and constructs.
Polaris is handy for the multiple gun puppies throughout the mission.
Shivermist is handy for holding the mortarfires still (they are only vulnerable to damage from the rear like trojans) and for stopping Ghostmane Stalkers (the invisibility-using gremlins). Letting someone with dedicated bomber gear take care of bombs for those is also an option though.
Gran Faust could be handy in some areas that are crowded, like the boss fight, for the knockback. It's probably better to take shadow over elemental weapons except for anti-turret weapons since constructs still get neutral damage from shadow but gremlins are resistant to elemental. It would also gain more from the ASI from your Vog.
I don't think I've seen Blitz often, but it might be useful for the boss fight against Seerus and the battlepods, don't remember if the pods are neutral or strong vs piercing though.
Once you have beaten the level once, you will have access to the hammer, which can be used for puppies and pods, and the bomb, which is handy throughout the levels for mortarfires, stalkers, and everything else basically.
If I have time, I might be interested in doing some tier 3 OCH runs (I already own it) for radiants and more mask pieces. My IGN is Hareth, I'm primarily a bomber. :)