It's like it say's y'all! My friends and I are doing a New Years (on saturday tho) One Shot Battle Royale at level 12. I'm looking for help coming up with a build for this because I usually play the tank in a party composition so I'm really unsure of where to begin. honestly open to anything. 

so far the only thing i had in mind was Moon druid 11 with a dip in monk, so i have SOME ac out of wildshape, take some support spells so i can help people out with their fights and hopefully not get targeted, and then go with a sunbeam into bonus action earth elemental so i can burrow in between my laser shots. alternatively i was thinking if i didn't wanna stay on the ground i could do call lightning and air elemental to pick off ranged combatants before swooping in close. 

HOWEVER

I'm open to other builds and ideas. 

If your goal is to have fun, then play whatever you will enjoy.

If your goal is to win, choose something appropriate for your arena and try to avoid combat until everyone else has expended their resources. In Critical Role, Scanlan started combat by casting Resilient Sphere on himself, which made him invulnerable for as long as he needed.

In a PVP battle royale, your best bet to win is to find a way to avoid combat against everyone else for as long as possible. It's also probably the most boring way to play, since you spend most of your time either floating in a bubble of invincibility or turning invisible and stealthing, or something similar.

Another good strategy is to make sure that someone else in the fracas is always a more tempting target than you. Drop  a faerie fire on the Bard early on, and you'll get a lot of other players focusing on them as much as possible, since you just gave them advantage on all their attacks against them. 

As for your build idea... if your priority is building AC outside of Wildshape, there's no need for monk. A Druid already gets access to medium armor and shields, so with just 14 DEX, a set of Half-Plate and a Shield, you can get 19 AC (unless your DM isn't allowing you anything outside of your starting Equipment). You would need max DEX and WIS to beat that score. Although the interesting thing about Monk is that class features still apply while you're in Wild Shape, so the one-level dip in monk has the potential to boost AC in wildshape, especially since you keep your WIS score even when taking other forms. This is useless on Earth Elementals, since they have terrible DEX so even with max WIS you won't beat their natural 17 AC, but assuming Max WIS your Air Elemental form would jump to full-on 20 AC, regardless of what your DEX is in your normal form.

This is useless on Earth Elementals, since they have terrible DEX so even with max WIS you won't beat their natural 17 AC, but assuming Max WIS your Air Elemental form would jump to full-on 20 AC, regardless of what your DEX is in your normal form.

so i guess here it would be a matter of whats more beneficial for me in the moment, taking the air elemental and flying and having 20 AC, or going with the earth elemental and having 17 AC and burrowing between attacks to avoid damage altogether outside of reactions, right?

i'm not particularly pressed about winning, I mostly wanna just cause some havoc. hence the sunbeam earth elemental idea. someone recommended i take barbarian instead of monk so i can rage when my concentration is broken and thats a very tempting idea as well. 

Honestly... the burrowing earth elemental sounds like the most fun, especially in terms of just creating unpredictable chaos for the rest of the players rather than just trying to maximize your chances of winning. Monk is still a decent alternate choice... Monks get the option of using a bonus action to make an additional unarmed strike after taking the attack action, so a Monk/Earth Elemental basically attacks three times... but it also depends on how much of a stickler for the rules your DM is. The Monk bonus action attack specifically applies to Unarmed Strikes, and the Earth Elemental's Slam is considered a "Melee Weapon Attack". Barbarian, meanwhile, gives you +2 to your damage rolls and uh... that's it, basically. The Advantage on Strength Checks and Saving Throws is nice, but Earth Elementals already have 20 Strength so they're in good shape on both to begin with. Unarmored Defense doesn't do any better with Earth Elementals than it does with Monk, since the EE's DEX is still so low. And EE is already resistant to bludgeoning, Piercing, and Slashing Damage (although magic cuts through it, unlike with Rage), so at that point the question is what do you think would be more useful: Potentially 4 extra damage each round, or a bonus action attack each round?

That said, if you're willing to sacrifice Sunbeam and take another level of Barb, I think Reckless Attack would make it worth it to really just sow chaos on the game board. Combine that with the Mobile Feat and you can burrow toward an enemy, pop out, slam them with two Slam attacks at advantage with bonus rage damage, then dive back into the ground where they can't target you to take advantage of your reckless attack to gain advantage on you, and the mobile feat prevents them from getting an attack of opportunity against you.

Rogue/Echo knight with maxxed out stealth. Arcane trickster and cast fog cloud. Just hide the whole time and annoy everyone else with the echo! Bonus points for Aarakocra or the Aasimar that gets flight.

If it's a team affair (which might be what you're doing because you mentioned helping people out with their fights)

Oath of the Watchers Paladin. 

Your aura will give you and your allies within 10ft of you a +3 to all saving throws (if you have 16 Charisma).  And you can use an action and your Channel Divinity to grant you and three of your allies (again assuming 16 Charisma) advantage on all Intelligence, Wisdom, and Charisma saving throws.  Your aura also gives them all a +4 to initiative rolls, which helps your party go first in battle.

If your biggest worry is enemy spell casters, the Oath of the Watchers is pretty darn good at protecting you and your party from spells.  Even if you have just a 12 in Wisdom, you'll end up with a +8 with advantage on those Wisdom saving throws against those high powered battlefield control spells from powerful enemy spell casters.

I would recommend the Interception fighting style to help reduce damage from attacks to any of your melee allies that stay by your side.  You also get immunity for you and your allies within 10ft to being frightened, heavy armor, extra attack, lay on hands, improved divine smite, a D10 hit die, and a decent 10 spell slots for your level 1-3 spells, including Counterspell.  

If it's a team affair 

its not, i just thought of doing that to play mind games and make myself less of a target 

That said, if you're willing to sacrifice Sunbeam and take another level of Barb, I think Reckless Attack would make it worth it to really just sow chaos on the game board.

i was thinking if i wasn't wholly attached to sunbeam i could use call lightning for a comparable effect, but i have no idea if the arena is indoors or outdoors or not