I ALWAYS just use my starter pokemon. I recently beat the whole game (including the elite four and alder) with just my serperior. It's possible but it just takes strategy. It was fun to think of strategies and made the game much more challenging and fun. When I fought alder the first time, my serperior was lvl 95 with the moveset leech seed, growth, leaf blade, giga drain. The battle started and alders accelgor kept 2 hitting me with bug buzz. I eventually got past it and then ran into a volcarona (4x resistance to grass) and it 1 hit KO'd me every time. I eventually got past him and ran into a bouffalant with a grass immunity. Not to mention he also had a escavalier. I think most people would have given up by that point but I looked through my tm list and changed my serperiors moveset to return/light screen/growth/double team and made it hold a bright powder. i just double team spammed and growth spammed to max and then preceded to sweep his team with return. That victory felt so good.