Its simple, just get few mana potions, idle in woods so they cant see you and let potm get xp/creeps. Use every opportunity when your creeps push to kill or send one hero to base. If you are on longer line pull neutrals each times its possible. Due the great surviving skill potm can easily handle 1v2 line. They will be mostly playing very defensively because of es/levi waiting for kill so potm will have all the space needed.
If line is pushed go gang with a tp or walk to a close line to get a rune and kill. Thats es/levi + potm strategy shortly :P