The slowness of the toxic needle isn't the worst part, it's how damage to knights is handled: after an initial hit, the knight is invulnerable for a short period to avoid getting infinite damage from, like, spike traps or something. This invulnerability period means that only the first bullet from the autogun-line weapons actually hits. (unless this has changed recently)

Snarble/nightblade is a good combo for going all swords. I wouldn't worry too much about getting gunslinger armor, the bonuses are pretty weak on the 3* stuff and guns are mostly support in lockdown. Ash tail is 4* so that would bump you to T3, so you're right to avoid it.

Pure bombing can be fun, but requires a bit of dedication and insane masochism (ask me how I know). Again no special armor is needed since the recon shield and a fully heated bomb gives you max CTR.

The only guns I've found useful in T2 lockdown are ones that can disrupt someone's movement with a status effect - Kilowatt Pulsar, Cryo/Volt Alchemers, possibly Magnus (I've only been on the recieving end of that last one, so I can't judge it accurately.) Antigua is good for interrupting someone trying to charge an attack, but you're rarely going to kill anyone with it due to its low damage.

yeah, you definitely want to upgrade to dusker the difference is enormous. Be careful though, neither of these will really protect you against nighblades/elemental brandishes. As for a second weapon the nightblade is a good choice, the snarble will be good for long, shield canceling battles and the night can do lots of damage in a short period of time because no one wears shadow def. The other best options for swords are, imo, a shock brandish (for ele damage and status effect) or a sealed sword (for lots of damage and reach).

If you choose to get a gun the only two I would seriously consider are the pulsar or the magnus (alchemers and antiguas can work but they're not as useful in as many situations), pulsar is great for blocking off areas of approach to capture points (because of the large projectiles) as well as firing into a mixed group of friends/foes for support (because of shock). I play mostly as guardian and and being bombarded with a pulsar while i'm fighting someone else is definitely the easiest way to disrupt the shield cancel with a flourish game that I play.

Magnus, which is the gun i use in lockdown is, in my experience, the best for actually helping you win a one on one fight. It is a great gun for disrupting a charge or hitting someone who is approaching you before they get into sword range because of the fast bullets, pulsar is pretty easy to dodge in a 1v1 situation because its bullets are so slow. It does take a while to figure out where to lead your opponent but once you get that down the magnus is very accurate. My favorite use for the magnus is in a drawn out 1v1 fight when someone breaks distance (either on purpose to recharge their bonus/run for health or just because both people happened to go in opposite directions at once), do a quick switch to magnus and pop em once before quickly switching back to sword if they're not dead or popping em again if they don't try to get close again. You have to be very careful with this because if you try it when they're too close you will probably get hit with a full combo and not be able to do anything about it. Basically you should be just out of range for a full combo of whatever sword they're using and if you're fighting a striker only do it AS they finish a boost (or if you see it break) not after or they'll most likely already be boosting again by the time you shoot. Overall it can be a great advantage against someone who thinks they have half a second to regroup before you're in fighting range again. The occasional stun can be very helpful too but it's not something you should count on. Sorry for the long post, i love rambling about my magnus, it's so satisfying to get kills with.

If you go with a bomb as a secondary weapon the best options are the shock and freeze bombs, maybe stun or fire.

Finally bought one yesterday and have been trying it out. What you lose in the Kilowatt Pulsar's ability to close paths and cover retreats, you gain in mid-range reliability and putting down runners. Much more offense-oriented than the KP, and there's nothing like dropping someone as they're fleeing back to their healthpads.

The best role for guns in lockdown is as a secondary weapon, some people have got full gunner to work but it's really hard. If one of them wants to chime in with advice on full gunning in lockdown listen to them but really I wouldn't recommend it. I would recommend the dusker stuff over the gunslinger stuff as you'll be using your swords more often than your guns.

Edit: Also Krysophylax, thanks, but believe me your shock bombing skills are far more frustrating than my magnus and me.