I'm gonna try to answer this. o3o
-Be extra careful when in battles. Make sure you always pay attention to what's going on. if you don't, one of your team members might die! D:
-ALWAYS stock up on potions, because If you run out of healing items at the wrong time, then you could lose.
-Also stock up on pokeballs. extra team members in a nuzlocke is one of the best things.
-Research gym levels. this helps a lot so you can train to the correct levels.
-Overlevel your team. this is kind of annoying though.
That's all I've got. if I think of anything else i'll edit the answer. Hope I helped though! :D