You true polymorph the boss into a frog.

Cunning Action

Dwarf. Have that feat that makes it so that you gain HP when you dodge. Be mostly barbarian. Have a ridiculous AC, and attack, dodge, attack, dodge. You’ll never die!

Violins.... without animate objects.

Okay...What were you responding to?  Cunning action does not allow dodge.  It allows dash, disengage and hide.

For your mechanic, I say... The violin is a spiritual weapon.  Cast at 9th level.

Deflect Missiles.

Oh. I thought it was that thing with monks that let them use ki to dodge as a bonus action.... in that case, moon Druids. Become fire elemental, dash to enemy, light them up, disengage and run.

also, clever.

Fight against all archers, and have an army of 300 monks using deflect missiles. Archers gonna die.

A warlocks familiar.

You only have one bonus action you know.  :)

Multiclass into at least Wizard 3, cast dragon's breath on them and cause them to attack and breathe fire every turn.  It uses its reaction to attack, and can take the action to breathe fire at your command without you having to use anything.

Sphere of annihilation.

Stand 120 ft away from a bag of holding, cast a 3rd level catapult spell as a 5th level Sorcerer with Distant Spell to fling it, open face first, into the sphere while being 210 feet away from the sphere to avoid any of its effects. Either you have destroyed the sphere, you have it in a bag of holding, or you have sent everything within 180 ft of the sphere into a random plane while staying a cosy 30 ft. away from the blast zone.

Edit: I think a portable hole might also work for this.

Gloves of Thievery

1 shield spell blocks the missiles entirely.

 response to gloves of thievery A level 11 rogue with +5 Dex with expertise in sleight of hand can never roll below 29 and a level 20 rogue cant roll below 33

How can i abuse a immovable rod

(Use enlarge/reduce if needed)

Grapple a dragon more than 500 feet in the air.  This'll cause it to fall.  With your remaining action before you hit the ground, activate it under the dragon.  Free 4 tons of piercing damage if done with the point toward the dragon.  And you won't fall unless it does, which will cause even more damage from falling damage.

Magic Missile

Take 10 levels of Sorcerer, wild magic origin, and then 10 of Wizard, Evocation school. Have a 20 in Intelligence.

First, cast Bestow curse, choosing the fourth curse, the one that causes all your spells and attacks against the target to deal an extra 1d8 necrotic damage, this then causes a Wild magic surge to happen and you roll a 33/34, making it so your next damaging spell has its damage maximized. Then cast Magic missile at 9th level, if you did everything right, this deals 55 damage with the base missiles (Maximized from the wild magic), then an extra 88 from the curse (Again, this is technically part of the spell, so it is also maximized. This is considering each missile as a separate attack, therefore it deals an extra 8 per missile), and an extra 5 from the evocation wizard’s ability ”Empowered Evocation”, for a total of 148 damage with a damage type that very few monsters have resistance against, and without a save/attack roll. Oh, and you can use two sorcery points to make it a quickened spell, meaning that you can also cast Fire bolt, for another 4d12 + 1d8 + 5 damage!

Command. You only get one word, what’ll it be? (You can say any one word, just make sure it has no alternative interpretations)

For command, say think to an ogre to paralyse it.

Beastmaster ranger.

Third level ranger, Beast Master of course.

Use a bonus action to cast Dragon’s Breath on your companion, and then have your companion breath the element, since the spell’s effect is not considered an attack, it doesn't take an action to command your companion to use it, then use your action to attack. Now you can use the Dragon’s Breath ability to deal 3d6 damage every turn of your desired type of damage, for free! Not insane, I know, but better than nothing. (If you don't have XGtE, a Youtuber called Zee Bashew made a great video about the spell)

Sovereign glue

If you want to be completely evil, use it to glue a gold piece to the ground.

If you want to abuse it, use eyebite on a low-wisdom monster like the Tarrasque, and choose the sleep condition.  Then immediately have an ally glue the Tarrasque's face to the ground.

(Also, just to note, Magic Missile doesn't use attack rolls.  The reason evocation wizard works is because you only use one damage roll for all of the missiles. Bestow Curse won't work with that.  Everything else will, though.)

Arms of Hadar

(It depends on the DM whether the curse would work on magic missile, I’d say it would, but it seems you would say it doesn't count. Also, Tarrasque’s have Legendary Resistance, so the Eyebite wouldn't work)

(You didn't make something with arms of hadar.  It doesn't matter if your idea worked or not, they get to choose something for the next person to abuse.)

Hit someone in the head with the pan OR put a pan on the plate and throw it in their face to blind them OR stab someone with the cutlery, so many things you can do

Critical Failures :), good luck

This was hard. What you do is you have a spell caster cast dominance on a creature to make it provide cover for your ranged weapon character. Then, if you roll a critical failure, your ranged weapon person will have a chance to accidentally shoot the enemy dominated. 

Wild shape.

Be a Moon Druid... That's how

Where the Dragon will be in Dragon of Icespire Peak ;) Only people who have DoIP will be able to do this

Ok. Going to the spot the dragon is and performing any of the other exploits above that could result in death of the dragon before intended. Or perhaps using this knowlage to kill someone you hate by sending them there using a quest or gias spell.

A piton (the starter item)

Turn it into an arrow, use it as a regular piton, use it to climb up and down the enemy's base which your DM prepared for all last night.

Abuse;

Blade Ward