Searching for powerful lvl 2 or 3 to go against my party

Hi, 

I’ve been playing DnD a couple times, I’m creating a new character and I have some questions! Basically, I’m joining a one day campaign, where we will all start as lvl 1 characters. The GM reached out to me suggesting I become a traitor to our group… the idea is I play the whole campaign with my group, but on the final battle I join the enemy side. Does this seem doable? I’ve never experienced with character vs character combat, I don’t even know if it’s doable? Since the GM doesn’t want me to die too quickly, he suggested I secretly start my character a couple levels higher, so that I don’t die too quickly. I’m searching for character suggestions to be able to be powerful in the last battle, but while staying balanced throughout the campaign, and still giving a winning chance to the rest of the team. I should probably take a martial class to be able to stay alive longer? I’ve just started considering my options and this is all overwhelming! Any suggestions towards characters, classes, anything balance wise or if all this is even possible would be greatly appreciated! 

Does the party know their is a traitor?  Aren't all of the character sheets shared?   Wouldn't the members realize you are higher level due to your to hit scores and your higher hit points?  If a mage, your spells you use as well.

A fitting class would be a Warlock.  Would this a be a 2014 character or one from 2024?   The 2024 character gets his subclass at 3rd level.  That results in a lot more abilities.

Yes you can do character vs character. I have never been involved and would not want to be involved in that type of game.

I’m using 2024, and I’m thinking of having the same level as the others during the campaign, but have a different character sheet I would switch to for the final fight. Thanks for the feedback!

Having a separate character sheet for the final fight would be cheating. If the plot is your PC turns against the party is must be your PC. Your former friends should know all the stuff you can do, including your strengths and weaknesses. Your character will be pummelled and not really stand much of a chance but that will be cathartic to the other players as a fitting retribution for your betrayal. Presumably there will also be some other enemy in the fight as well, it won't just be your character vs the party.