Battle tree training?

l am making a team for taking on the battle tree in pokemon sun ,and l was hoping the more experienced Competitive players could give me advice on the best way to quickly level up and gain good evs?(which l barley understand).(l have ways to have all "best" ivs,so l dont need help with that part)

the team l am thinking of useing is venasaur (timid with toxic/leachseed/ingrain/leftover stall) and a lapras for whenever the enemy uses the same pokemon (which has happend in the past) so any advice on the 3rd pokemon for my team and the best way to train them would be great! like should l use the pokemon in battle or does just haveing it get xp from the xp share work? or for venasaur (which l am in the process of hatching) whats the best way to make it fast and healthy/defensive? (meaning train its speed,hp,defence/sp.defence,as i stated,it wont be needing attack or sp.attack)

edit:ok so l found this ign post which l think answers my questions.