While I understand that reason, it's still utter bullshit, hear me out. He has been in this body for what 3-4 months maybe more but in every major fight he is stopped short cause "it's not his og body and the body has never trained" but he never actually trains it after these fights to not hit these walls every single time. Maybe this is an example of him being naive but that attitude also got him killed in the last world, which in the story so far seems he hasn't grown past.
Sigh, like I know he wasn't going win this fight but seeing that none of the villains are scared or suprised as each of their allies are getting killed (though in my guess because of the story he killed none of them) even when they are working together to hold back the MC. Also as a side note I really dislike the trope in "new body" genre where the MC refuses to live how they want in this new body because "that's not what the og body may have wanted", like they're dead or gone somewhere else and by the nature of these stories they may have been Reincarnated too just like the MCs.
What Takeru Classic has done so far in this fight is the result of his training in the new body. Chapter 8, he's doing jogging and probably calisthenics. It's not like Original Takeru could withstand being smashed, even with Healing Magic. He's specifically said, he doesn't want to give Original Takeru back a distorted body. This isn't Wrong Way to Use Healing Magic, where he can become super strong. He doesn't have the Healing skill to do that sort of thing.
Look up on YouTube "Vince McMahon torn quad". Besides being hilarious, You can literally destroy your body, if you're not careful. In Vince's case, it's from Steroid Use. But the same could be said of uncontrolled Strengthening Magic Use. His original Strength Level was from literal Years of training.
And the Demons could be unfazed by the vicious assault because they have a Skilled Non Combatant Medic/Healer. Takeru Classic, knows that he can't leave Demons alive, Chapter 5 Page 7. But he also knows, he has to save some strength and not exhaust himself from fighting the pawns. So he's not wasting time doing a coup de grace, because he has to worry about who's coming next.
Anyways, Takeru was /never/ going to win this fight. Because the Demon Lord could just teleport him away. After he exhausted himself on the minions, he still needs enough strength to fight the Demon Lord.