@TheGoodFella I think he didn't go all out either to assess the demon's strength or to not get the three adventurers who came with him in the crossfire, or both. Since after the slime took the three to a safe place he went all out
About "why not from the beginning" - one handed on dragon oni was target moving with speed of said dragon and limited maneuver aka sitting duck. With his power it's entirely possible that he would've dodged fire tornado when on land and healthy.
Watch him not get any credit for this cause F rank is in title, at least remain F rank but let people know him as the strongest in the city but chose to remain F rank or something so we don’t have to waste our precious chapters dealing with annoying bullies!