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

3 legendary and 5 non

i am leaning towards 5.5 elemental monk

I'd start by reframing the question. Your goal shouldn't be to build the best character possible but to build the best party possible.

The best individual characters are often generalists who cover a lot of different tasks. The best parties are normally composed of specialists who do what they do extremely well.

Some key elements:

• Win Initiative. High level enemies have tremendous Initiative scores. This enables them to dictate the terms of the battle. However, Dex-based characters with features like Ambush and Alert back by weapons of warning or Stealth can give you a very good chance that someone in your party will go first - and Alert allows that someone to pass off their winning Initiative to the party member who can themselves dictate the terms of battle (normally a caster with environmental effects).
• It's better not to roll than to roll well. Crouching behind full cover makes you immune to most targeted spells. Standing in the open even with an exceptional save will always be worse than that. Likewise, most DC-based all-or-nothing spells aren't very efficient because of how often the result is 'nothing'.
• Hit is a lot easier to buff than save DC. It's possible to build high level characters who effectively never miss - even against high level enemies. It's almost impossible to pile on enough bonuses/penalties to make your save DC that reliable. About the only save DCs you should be comfortable with are those which still have effects you can justify spending an Action on even if the target saves.
• If you plan on going into melee, you'd better have to a plan to not stay in melee. The sort of toe-to-toe fighting many players are comfortable with from their low level campaigns is normally a terrible idea at high levels. You want to get in, do your thing and then get back out to safety. Sticking around so an enemy can simply unload on you - even with a great AC, resists, etc. - just burns party resources.
• Every character has an Action, a Bonus Action, a Reaction and Concentration. Use all of them. A lot of builds have an abstract notion of "I can do a lot of things" without a clear focus on what they'll usually be doing each round. While you'll vary what you do - casters in particular tend to have a first round (setting up the battle) different from what they do the rest of the time - but you should have a basic outline of how you're using those resources every round rather than a vague idea of "I cast a spell".
• Magic items replace petty spellcasting. At level 6, your Paladin being able to cast Cure Wounds is valuable. At level 20? You've got magic items that cover that sort of thing. When planning a level 20 build, knowing what magic items you'll have and how they transform what you can do is essential. It's not just +3 Swords.
• Control movement and vision. The key to winning fights is to set the terms of those fights. Slows are particularly effective because they normally don't involve saves - having an entire party that can each stack -10 Speed very quickly cripples most opponents. Being able to create opaque barriers to hide behind means your opponents are just stuck trying to maneuver around those opaque barriers (slowly) to even attempt an attack. On the flip side, your own party members need to be able to function in such an environment - high mobility, Blindsight, etc.

If I had to pick one class I wouldn't want to go into a level 20 adventure without, it would probably be Bard. Bard not only ensures you've got access to every critical high level spell but Bardic Inspiration is probably the single best core class mechanic.

Monk is very powerful at high levels but so are most classes. Treantmonk did a nice video showing that Cloud Goliath level 20 Dex-based Fighter is basically unstoppable due to the combination of Cloud Goliath's teleporation, and Indomitable. And can put out ridonkulous damage with a Longbow with Great Weapon Master and Action Surge.

If you are going full martial (be it fighter or monk) you should definitely go with Cloud Goliath for that free non-spell BA teleportation. Also grab yourself a Cape of the Mountebank and some other magic item that gives you free flight, like Boots of Flying. I'd also highly recommend the Cloak of Invisibility as one of your Legendary items. 

Honestly with that number of magic items, your class doesn't matter a whole lot. What will matter more is your feats / epic boon, and your magic items - especially if you go martial. So here's a magic item list for you to make a broken martial character:

Legendaries:
Cloak of Invisibility (A)
Belt of Storm Giant Strength (A) 
Cape of Enlargement (A) 

Non Legendaries:
Force Breaker Weapon / +3 Wraps of Unarmed Prowess 
Cape of the Mountebank
Figurine of Wondrous Power (Bronze Griffon)
Ring of Resistance (Necrotic)
Ring of Resistance (Force)

My lvl 20 Warrior of Elements with 105 movement (before Haste) and grappler feet just last week got a hold of a Lich king in a grapple and used his bonus action to disengage and dash straight up for 4 rounds 880 feet (making all his saves from the desprite Lich King trying to charm him) before just letting him drop in free fall. Along the way up he used all of his attacks and Mage Slayer to break all concentration.