Yow. Powerful!

Very odd combination of abilities though; doesn't really seem to have a theme. Obviously deathtouch for a scorpion makes sense, but then it could also be really big which doesn't normally combine; first strike again feels scorpiony, but why does it gain hexproof? It just feels kinda "My trigger activates? Throw the kitchen sink at them!"

I knew I wanted this mythic to be a creature. Originally it was going to a djinn, but then it felt weird not having flying (although I already made one). I still liked the idea of being powered-up by burning an opponent (or yourself) as that seemed pretty magical. I didn't want another grounded efreet, and the marid haven't really been characterized in green, so I thought I'd make some kind of animal and such would be okay (since animals have had all kinds of odd abilities). I didn't want to give the creature trample and though hexproof worked better to give the creature protection when it's going in for the swing (or potentially block). Scorpion was the first creature that came to mind, so I gave it deathtouch fit. Deathtouch can also be relevant for when the creature isn't big I think. I did consider two other creatures: caracal (cat) and goat, but I wasn't really feeling either one at the time. Additionally neither would make sense having deathtouch. Now I guess I could have made this a manticore.

Indeed; my point was deathtouch is relevant on small creatures. This is aspirationally 8 power or more - not small!

Nothing wrong with it as it is really; it just feels kinda "grab bag of abilities" rather than "Here is a thing, that makes sense as a singular thing"

I quite like this. First strike + deathtouch is a great + combination (going all the way back to Voracious Cobra), +5/+5 is hilarious and enough of a reward to get both Johnny and Timmy happy, and hexproof is an intriguing option and a handy way to protect itself (if you have instant-speed player-burn available).

Best thing to do with this? I think I like Grapeshot or Pyromatics. Fun self-damaging options include Char/Psionic Blast, Hail Storm, Orcish Cannonade, Aftershock/Ashes to Ashes. You can trigger it twice (or N times in a multiplayer game) with things like Flame Rift, Slagstorm, Price of Progress, and of course Earthquake and Hurricane.

I think this is in an excellent spot for a mythic. Even if deathtouch is mostly irrelevant, it still fits the scorpion flavour, and it lets this be handy against the occasional 20/20 that turns up in Commander games.