as far as i know Nature Training ranking does not depend on who finished first the challenge

it depends on how fast (or how early in the round) you finished the challenge, for example Level 50

lets say Player A finished the challenge in 4 rounds first, he will get the 1st ranking

then Player B finished the challenge in just rounds, so Player be will take the 1st ranking

i've tested with friends, initially i got 1st rank.. then he who had double of my BP finished the challenge & take my rank

unlike Strong Approaching, like you said.. it depends on who finished the challenge first