The Grinch

funny enough, treecko is actually the best starter for emerald nuzlockes. fairing well against 50% of the evil teams, most of Tate and Liza's mons, the first, third and last gym and the champion. only really strugeling against Winona, Flareny, team magma, and 2 e4 members

the problem with sceptile is that what it doesnt beat into oblivion, it struggles super hard against. also, sceptile isnt particularly that good against wallace, either. sure, it easily beats whiscash, but if you go over the rest of his team, sceptile really has no reason to engage gyarados and tentacruel (good spdef, no super effective attacks), and wailord and milotic are risky because while theyre bulky, they also carry blizzard and ice beam, respectively. funnily enough, ludicolo is the second-easiest mon to deal with for sceptile, due to it only knowing surf and giga drain as offensive moves, and sceptiles immunity to leech seed.

sceptile takes up the role of glass cannon, but what a glass cannon needs to do is ohko stuff left and right. sceptile cannot do that well because there are so few things that it can hit for super effective damage. and, I assure you, counterattacks will hurt sceptile, due to poor typing and poor bulk.

compare that to swampert, who fills an unique role with its water/ground type. it learns earthquake -an absolute godlike move, by the way- through leveling, gets easy access to its second stab in surf, and absolutely walls everything in existence ever. maybe I look a bit too much like a swampert fanboy right now, but you literally cannot imagine how great the species is in a nuzlocke. its quite possibly one of the best pokemon ive ever used so far, bar maybe gyarados in sinnoh and excadrill/darmanitan in unova.

for reference- the info page of the mudkip line in rse on project nuzlockedex right here.

http://project-nuzlockedex.wikia.com/wiki/Mudkip_Line/RSE