I've tried T3 och a few times, and I keep getting horribly murdered on the boss. A lot of it is because I play solo, but I think another part of it may be my gear choice.
You see, I'm still a defender elite. I still need to get a 5* shield, which will take quite a bit of grinding to get. It is also important to say that most of my 5* stuff is unheated.
Here's my loadout:
Vog Cub Cap
Vog Cub Coat
Heavy Plate Shield
Obsidian Edge
10 Sparks of Life, if needed
???
About the ???, I'm wondering what my second weapon should be. Here is what I've been considering:
1. Faust
2. Winmillion
3. Gigawatt Pulsar
4. Master Blaster
5. Electron Bomb.
6. Freezing Vaporizer Mk2
I'm willing to use the Sparks of Life I have, but it would be nice to use as few of them as possible. I could kill Seerus pretty easily with 10 Sparks, since I killed him about halfway without any last time.
So, which would be the best secondary weapon?
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.