He doesn't need to evade it, nor does he need to use chakra to maintain the jutsu... the link between Asuma was never lost despite being decapitated and his chakra flow halted. Not that it matters as nothing is no wound external or internal is halting Hidan and his Immortal Body.
They could never touch him anyways :lol
I wont lie, i dont even remember Hidan being decapitated before he killed Asuma but we DO both know if that were to happen he wouldn't be able to move. He needs someone to put his body back together so if hes left there he dies of malnourishment since the ritual will ware off within a a day since we know hidan has to perform a ritual often to maintain his immortal body. His charka network is still that of a normal shinobi and he still feels pain so immortal or not, i don't see hidan winning since hes one to take his immortality for granted. Especially against Genin.
He was avoiding Asuma with an older shikamaru's help
And Neji has proved time in and out that his speed feats are formidable as a Hyyuga. In close range, Neji gets one hit and Hidan is at the very least slowing down. Hidan has proven hes fast but in comparison to a jutsu like 64 Palms, hes guaranteed to get hit.