OK, i am a deus ex machina hater but this isn't so bad, it's mostly bad writting.
1) The new enemy was so ridiculous strong we all could tell only deus ex machina would save them or the manga was going to end (i thought it was an incoming axe so i am kinda happy we got the crappy deus ex machina);
2) Time travel isn't new in this manga, MC isekai'ed with time travel so the way they used it to save the manga didn't come out of nowhere, also MC didn't power up from it since he had no control over it, it was like a burst of his magic power triggering some hidden stuff, bad but at least we didn't get any random god saving them etc. It was a smart way to save the MC since he was one of most powerful character in the manga world anyway;
3) This is bad writting, it's a dumb arc, it came from nowhere, it's completely pointless and just left the readers angry, i hope the author can still save the manga because i like the characters.
And please MC, stop the Red magic nonsense, i thought all this isekai stuff was about MC finding out Red magic wasn't his strongest magic, i hope it's all fixed now, before this arc MC was training his other magic then he suddenly forgot all about it and aimed for the strongest Red mage, wth.
It's like the author left the manga for an arc, someone else wrote this dumb stuff and now the author is back and had to reset everything because it was beyond salvation.