I have recently read in some threads that it is not worth having the one Leviathen blade in Tier 3.
This worries me as I have used the calibur up to 4 star
I was thinking that if I get the Glacius and the Dark thorn blade, This would mean I cover everything other than Jelly's and gremlins (With Storm Driver as my gun).
If I were to use this, would it be good for tier 3?
Should I get a Shadow Sword/Gun to cover all grounds?
Or should I Switch to a Piercing gun?
I'd say use some sort of elemental gun (Storm Driver is fine, although you may want to change to a Polaris or Nova Driver later on). Elemental damage is the only one that's extra powerful on non-dodging enemies. Shadow has dodging gremlins, Piercing has dodging beasts AND fiends :/ Elemental damage is good against constructs (some use a shield) and undead which don't dodge your shots.
On the note of which sword you get, I'd advise piercing and/or shadow. I assume you prefer having a gun/sword loadout, so you'd want to get swords with a different damage type than your other weapons.
If you go piercing, go with the Barbarous Thorn Blade of Final Flourish, the only difference is in the charge attack.
If you go shadow, it's a choice between an Acheron (3-strike, more defensive due to quicker shielding ability) vs. Gran Faust (2-strike more offensive due to wide attack spread and slower shielding ability, also inflicts curse). IMO they're equally effective in the right hands, I have both, and I prefer the Acheron because I'd rather kill a bit slower and be safer :P
If you're going to do Vanaduke runs, a Blitz Needle is a great piercing gun for taking him (and Trojans) down, but sadly not much else.