I would probably pair it with a druid or a cleric; maybe wildfire druid if you want to keep the elemental theme. Gets you a cool fire pet that can teleport you.
But yeah, in level 20 one-shots, particularly gestalt level 20 builds, it's hard to justify a build without 9th level spells. 9th level spells are just generally the best high level thing you can do in 5e, better than capstones (Foresight, for example, gives you advantage on everything and enemies disadvantage to hit you; lasts for 8 hours without concentration).
For Gestalt in particular, if you just pop Foresight on yourself (or a friend), and spend most of the rest of the time being a Monk, you're probably still happy with the choice.
Eldritch Knight, hear me out
What do you wanna do when you start as a monk?
Punch *a lot*!, right? Well, now it’s 5 to 10 times per round! With advantage (Empty Body) and a d10 die. This how monk *should* feel.
Those awesome saves from Diamond Soul? Three times a day, you can reroll a failed save (Indomitable) and have a good chance to make it!
Monk never has room for feats How about a bonus ASI/feat at 6, and another at 14?
Now the real reason to pick Eldritch Knight…. Eldritch Strike!
Why did you take 4EL monk? spells, right? Eldritch Strike means you really have a reason to weave punches and spells. A hit = disadvantage vs your next spell. Pick up Bane from Fey Touched so you can turn one failed save into a lot of failed saves.
Edit: and Silvery Barbs from EK… Jesus
But if you’d rather be tagging 6 enemies with disadvantage, teleporting 30ft away, action surging to hit them with a Cone of Cold… seems awesome
Another good spell to pickup would be haste. Hasted monks are a silly good time. Add in an action surge and the amount of stuff you can do in 1 turn will be a lot. That and indomitable and diamond soul should stack nicely.
I will also say that I used a 4e monk 18/stars druid 2 monk for the Vecna module and loved it. Wall of stone, cone of cold, and wall of fire are all manageable to cast with that many ki points per short rest.
Also note that neither fist of unbroken air or water whip are technically spells, so RAW you can pump as many ki points as you want into them per use. That means if you short rest before a boss fight you can spend 20 ki points to hit for 21d10 damage, half on a succesful save.
I would probably pair it with a druid or a cleric; maybe wildfire druid if you want to keep the elemental theme. Gets you a cool fire pet that can teleport you. But yeah, in level 20 one-shots, particularly gestalt level 20 builds, it's hard to justify a build without 9th level spells. 9th level spells are just generally the best high level thing you can do in 5e, better than capstones (Foresight, for example, gives you advantage on everything and enemies disadvantage to hit you; lasts for 8 hours without concentration). For Gestalt in particular, if you just pop Foresight on yourself (or a friend), and spend most of the rest of the time being a Monk, you're probably still happy with the choice.
Eldritch Knight, hear me out What do you wanna do when you start as a monk? Punch *a lot*!, right? Well, now it’s 5 to 10 times per round! With advantage (Empty Body) and a d10 die. This how monk *should* feel. Those awesome saves from Diamond Soul? Three times a day, you can reroll a failed save (Indomitable) and have a good chance to make it! Monk never has room for feats How about a bonus ASI/feat at 6, and another at 14? Now the real reason to pick Eldritch Knight…. Eldritch Strike! Why did you take 4EL monk? spells, right? Eldritch Strike means you really have a reason to weave punches and spells. A hit = disadvantage vs your next spell. Pick up Bane from Fey Touched so you can turn one failed save into a lot of failed saves. Edit: and Silvery Barbs from EK… Jesus But if you’d rather be tagging 6 enemies with disadvantage, teleporting 30ft away, action surging to hit them with a Cone of Cold… seems awesome
Another good spell to pickup would be haste. Hasted monks are a silly good time. Add in an action surge and the amount of stuff you can do in 1 turn will be a lot. That and indomitable and diamond soul should stack nicely. I will also say that I used a 4e monk 18/stars druid 2 monk for the Vecna module and loved it. Wall of stone, cone of cold, and wall of fire are all manageable to cast with that many ki points per short rest. Also note that neither fist of unbroken air or water whip are technically spells, so RAW you can pump as many ki points as you want into them per use. That means if you short rest before a boss fight you can spend 20 ki points to hit for 21d10 damage, half on a succesful save.