So, as long as I've been playing I've never actually ran a SL yet; the price+procrastination=not bothering.
But now that I've got myself all my gunning gear finished up and am ready to roll as soon as I get my people ready, I've hit a snag; the shield!
Now, everywhere else I just use my shiny Swifty, including FSC and DMs and do just fine, but since I've not been to a SL I'm not sure how appropriate that would be (and don't want to find out that it's a bad choice after I've been locked into the mission).
The other choices would be BTS and DSS.
Swifty:
Yes: Increases attack speed of all my guns...a lot
No: Lack of any meaningful defence, and I suspect I'll be using more charges than not...but not sure
DSS:
Yes: Shadow defence and poison shielding sounds juicy
No: No piercing defence and I suspect there'll be more of that to worry about, at least in the boss room
BTS:
Yes: The boss room will likely be mostly piercing and helps for all piercing before that, too
No: No poison resist or shadow defence, and the sword bonus is useless
I'm actually at a total loss for which to choose here. Any advice would be greatly appreciated~ :)
Since I haven't tried gunning in an SL, this is only from my experience as a Swordie.
SSB: As far as I remember this is the only shield which directly benefits gunners. In UGWW, the difficult parts mainly come from small area arenas with tons of enemies thrown in, with a mix of shadow and piercing enemies. If you are really really good at dodging, which you might be since you DMs with SSB with too, you could consider this an option, but not the best one imo. SSB breaks too fast, and you wouldn't want to take risks on your first SL trip, eh? :3 Go with it only if you are really, really confident you can keep up with it. You could try watching an SL run on Youtube if you wish to assess the risks before going there.
DSS: Most people prefer shadow damage shields for UGWW, and I, personally being mortally afraid of Kats >.> used my Crest of Almire there. As it is, beasts aren't really much of a danger anymore thanks to Alpha Wolvers being nerfed. And since gunning will keep you mobile all the time, you have all the less to worry about beasts. So shadow seems to be the number one option here.
BTS: Since you already stated, as a gunner the sword bonus will be useless anyway, and thanks to the vast majority of UGWW having Shadow damage Howlitzers, Zombies, Kats it won't help much against them anyway. As for the boss, afaik, only the spin attack spikes do pure piercing and the Snarbolaxs' bites do huge amounts of normal damage. They usually (almost) break every shield with their 3 bite attack, unless it's a plate series shield. So bye bye to SSB here, you will pretty much be defenseless during the boss fight. BTB is a better option for the boss than DSS, but it will probably weigh you down for the entirety of remaining of the run.
TL;DR: Take the DSS, best, imo.