Need OP ideas for pure 5.5 lvl 20 ideas for power 1 shot.

Playing a 1 shot that is 5.5e lvl 20. Dm said bring your best and OP builds because it will be super hard. Mostly battle. .please help. I don't like to play full casters. But open to all ideas. Also prefer full class builds. 

There are two significant problems you will run into with a non-caster build.  First, if your DM is not extremely experienced running level 20 games, it is very likely they will not know how to burn a caster’s resources in a manner that is balanced, but mitigates the divide between casters and martials when at full resources.  Second, martial builds are extremely dependent on magic items to keep up - they want things like weapons that deal additional d6s of damage on every hit to take advantage of multiple attacks.

If you want a non-caster build that keeps up, level 20 Fighters are extremely consistent, particularly if properly equipped.  4 attacks per round, with the possibility of action surge, is nothing to scoff at, and things like Indomitable help mitigate disaster.  Battlemaster has a number of small, potent combat tricks, Echo Knight has some damage boosts and pit of combat utility, and Cavalier is the  best at holding the line if your other players are casters or at range.

Monk is also quite good at level 20.  Stunning Strike is the single best way to burn through legendary resistances in the game - a necessity if you want to clear the way for your allies to do flashy effects.  They also get a significant number of attacks, and though their item selection is more limited than Fighters, there do exist a few ways to increase their damage per hit.

What kind of magic items do you get? It can make a big difference here.

Monks eventually get proficiency in all saving throws.

You will probably want the wraps of unarmed power.  There is a magic item tattoo, (inkwell? Maybe?) That lets you punch from 10 feet away so you dont have to physically touch things you attack.

Hang on. If you want op, go pure artificer.

The level 20 capstone basically gives you a bunch of ways to cheat death.

But more importantly, an artificer can create magic items from a liat of plans, and by level 20, this gets crazy.  You can build 6 after a long rest, and there is a feature where you can swap to a different plan you havent built yet and build that. Summoning items you can build include:

Horn of valhalla(brass)

Bowl of commanding water elementals

Brazier of commanding fire elementals

Stone of commanding earth elementals

Cube of force

Cube of summoning

Daerns instant fortress (can you still drop it on enemies?)

Figurine of wonderous power (griffon)

Figurine of wonderout power (lions)

Horn of the endless maze.

Also, at level 11, you can create a spell casting item which lets you take any artificer spell in your spell list, up to LEVEL THREE, put it into an item, and then you can cast that spell 10 times.  If you play the artillerist subclass, you get fireball added to your list, meaning your spellcasting item can cast a level 3 fireball ten times.

If you player armorer, you are basically ironman, you will have an armor class around 24, plus cast mirror image for the occaisional hit that slips through. And your armor has weapons built in that use inteligence (artificer spellcasting ability) for tohit and damage. Armorers dont get fireball, but they do get lightning bolt and can put that in their spellcasting item