I've narrowed down my secondary weapon to either Gigawatt or Master Blaster.
The obvious drawback of the Giga is that it can't kill gremlins, but would it be a viable choice against the gun puppies in the earlier depths? And either way, I could just use OE charges to kill the gremlins, so the gremlins don't really matter.
Also, if I take the Master Blaster, should I ever have to use it at all? I mean, I could attack through the barricades with my OE and deal a lot more damage than a MB, right?
A third, but undesirable choice would be to grind more and get more gear. However, would OE/GP or OE/MB let me finish in a reasonable number of sparks?
At this point, I'm going with OE/GP
It's no big deal, but this thread is about equipment in Tier 3, so it's probably better for Arsenal forum than for New Recruits. Consider moving it (by editing the original post and choosing that forum from the drop-down menu).
As you probably know, Heavy Plate Shield is not a great choice. Gremlins inflict normal and elemental. Grey Owlite is a great choice for defense. Barbarous Thorn Shield and maybe Swiftstrike Buckler are more offensive choices.
Your Obsidian Edge is a wonderful choice. Vog Cub armor is also quite appropriate.
I don't know whether Seerus is freezable or vortexable. If so, then those bombs might work. Faust and Winmillion are definitely outclassed by Obsidian Edge here, so don't bring either of those.
My recommendation is a shadow gun. (There are no shadow-resistant enemies, and most enemies are gremlins, so it's okay to carry only shadow weapons.) You can use it to take out turrets from afar, although not as quickly as with a good elemental gun. More importantly, you can use it to wear down Seerus in the boss fight. If you can't spend any time to get good, or if you are pretty laggy, then consider Sentenza/Obsidian. But Umbra Driver and Winter Grave are both more powerful and more fun in the long term. Grim Repeater and Permafroster are probably okay too. I'd recommend against Biohazard for your first shadow gun.
If you meant us to pick only from the weapons that you mentioned, then I'd say give the freeze bomb a try, then the blaster.