Unfortunately Guacamelee! Super Turbo Championship Edition do not support 21:9 aspect ratio natively, ultrawide resolution can not be selected in option menu. But we can force engine to render extra image in wider resolution by HEX edit. That solution is just same one like in Guacamelee! Gold Edition, but its not listed in PCGameWiki page of G!:STCE. It was pointed out in steam forum by user "Sv. Prolivije"

1. Download Hex Editor program of your choice, for example i will use HxD (Official download link[mh-nexus.de])
   
2. In HxD itself, open game.exe file from game directory folder.
3. Press CTRL+R (or find > replace), change data type to Hex value, in "search" type: "39 8E E3 3F" and replace with "CD 90 18 40". Change direction to everywhere, and click "replace all"
   
4. Accept one changed value and click Save icon (Floppy disk). HxD also will create *.back file just in case, now you can close program, that is all.
   

From that point engine going to render everything with extra image on the side of the screen (Hor+), without any glitches. Menu and all 2D HUD elements are fine. Support seems to be flawless.

Game is fully adjusted to 21:9, playable and enjoyable on wider resolution.