Rate/Improve my Pokemon White 2 Team!

Some of these might involve move relearner!!!!

On Lucario, scrap what you have and go for a special set, given the nature raises that. Dark Pulse / Vacuum Wave / Aura Sphere are your long term options. You can keep Quick Attack for the extra priority if you want, or any other attacks. I'd say Nasty Plot but it's no use in-game, so just move relearn those, if you can, and you're good, or otherwise learn them through level (or if it has Hidden Power Ice, again for Flying types - read about that @ bottom).

On Azumarill go for Waterfall once you get it > Aqua Tail and Aqua Jet > Bubblebeam, and the rest should work fine in-game given that it is a primarily physical attacker.

Too many STABs on Serperior. Go for something like Leech Seed if you want > Leaf Tornado as it's constantly reliable (my Head Smash missing in-game is extremely annoying on Emboar and relying on accuracy isn't good because the opponents always hax you sigh!) and go for Leaf Blade as what you're primarily using with Mega Drain to recover - that allows you to have multiple STABs and with the nature not affecting that you're all good. There's not really much coverage Serperior needs in-game tbh.

On Sandile Earthquake > Sand Tomb, and Stone Edge > Rock Tomb but keep that if it's easier. You can keep Swagger but it won't really be of much use tbh, but otherwise it works.

On Sigilyph... there's not really much you can do so it'll work! Also check Serperior's Hidden Power and later on obtain the TM as a PWT prize iirc, if it's something useful like HP Ice, so you can take out Flying types although with a -speed nature chances are that won't be happening much. ;(

Sorry this is short :(

Your team needs a fire type. maybe trade for an emboar or one of it's previous evolutions on the GTS, but, If you don't want to go through the trouble, I highly recommend Darumaka/Darmanitan. It's a great offensive pokemon that can deal out tons of damage.