As a level 13 wizard, prepare for a fight in the following way. Step 1) Have a portable hole. Step 2) learn the glyph of warding spell. Step 3) cast glyph of warding onto the walls of the portable hole with spell storing. Store the following spells (requires multiple days): Tenser's Transformation, Tasha's Otherworldly Guise, Mirror Image, Blur, Haste, Mage Armor, Enlarge/Reduce. The glyph trigger is a special command word or phrase that you choose, such as "it clobbering time."
Glyph of warding allows spells with concentration to linger for their full duration without conflicting with other concentration spells. These spells are active until they end and the concentration cannot be broken.
Portable holes do not end glyphs of warding because the extradimensional space within a portable hole specifically has continuity (used as an inventory and can contain creatures while closed) and does not move when the opening to the hole moves after being folded up. It's a 6 foot wide by 10 foot deep cylinder that exists on a different plane that can be accessed from anywhere you put the door to it. For flavor, even if it's orientation changes relative to where you put it down (wall, floor, ceiling) the space itself hasn't moved though the shift in gravity might cause some objects within it to move when you open it. Even if you specify that the cylinder itself can't have a glyph inscribed on it, be ause it doesn't exactly have walls so much as spatial dimensions, objects within the hole can have glyphs and would be hard pressed to move beyond the glyph's 10 foot spell end limit in a space that is only 10 feet long.
Stacking Tenser's Transformation, Tasha's Otherworldly Guise, and Haste allows you to Make two attack actions on your turn, and each attack actions makes four strikes--each one with advantage when done with a simple or martial weapon, also dealing an additional 2d12 force damage. Also you make them using your spellcasting modifier. Transformation and Otherworldly Guise stack because the additional strikes per attack are are "benefits" not features or abilities; having written and edited homebrew for years now, there 100% is a difference and it does matter in the way you phrase things.
Combine this with all the ac buffs, blur and mirror image, plus damage resistances and temporary hit points; you virtually cannot be killed and you make 8 attacks per turn plus bonus actions and any reaction you make. This is also assuming you aren't wearing any magical items such as enchanted weapons and armor.
All you have to do is, before you fight, complete step 4) put down the portable hole and activate all of your glyphs at once.
The only balance to this is that setting up a portable hole this way requires multiple days with many hours of spellcasting, including using a 6th and 7th level spell slot every time you prep Transformation and Otherworldly Guise.
To be clear, this is not something I would ever do (perhaps not more than once) and not something I would let a player spring on me. And even if I did let a player get away with this, I would cripple them with exhaustion after the spells ended. This is a rules based thought experiment.
Edit: I hereby dub this strategy "the magical girl transformation chamber"
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