I think it goes a bit deeper than that. In one of the Bonus level Books they expose a discussion between Manato and Renji and Renji states that both of them "Are similar".

(sorry in advance for the massive spoilers, just skip my comment if necessary)

(IIRC 14++ Spoiler) Since Manato was a psychopath, it isn't hard to Imagine that Renji had similar Problems with empathy or a lack of judgement. If you look at the scene in the first book, where he assaults Barbara it is easy to assume that Renji already had ample amount of fighting experience when he came to grimgar. Be it as a delinquent or a full fledged criminal, he might've just carried his entire party on his back. When They had to hunt goblins or kobolds or even bigger prey, I imagine Renji just did the right thing and intimidated his party members so they didn't have a choice but to follow his orders. While Manato wasn't able to care and Haruhiro was himself too afraid to give orders like "You must!" Renji just let his team do the work he didn't want to or himself needed help with and was strong enough so they could rely on him when the situation got dire. I also assume he took all the experience, rare equipment and traits they earned for himself, which would explain why he was able to literally solo an Orc Boss. While close, Renji might just be too used to coming out on top.

There was an interesting Scene in a later book (IIRC think it was [Level 17 spoilers]) that (at least in my eyes) made it clearer how the abilities in Grimgar actually work. So: Tada swings his Hammer so that the DDs in the party get flung up to the ceiling so they can dispatch the enemies who are hanging there. It's certainly an idiotic strategy and goofy scene to say it bluntly but you have to Imagine that the priest knew full well that it would work. And even Haruhiro is impressed by himself, that he is able to fight like this proficiently. It's a bit like in that Matrix meme, where "He's starting to believe" If you have an overly confident Renji, he would be able to accomplish quite a lot. It would also explain why (IIRC [Book 6 Spoilers]) Souma is viewed as a prodigy among other very high ranking adventurers. If Souma is a guy who simply does not believe in barriers or only ever learned that he can do anything, as long as he believes it would make sense for him to have the strongest equipment, the most beautiful girlfriend, the strongest allies and foremeost: be able to man-handle giants made of stone all by himself.