I have defeated both guys with many pokemon combinations, but there's one tactic that I use that always worked.
-Don't attack his lileep, try to kill all eeveelutions first before facing the green beast;
-Make sure the lileep is confused or sleeping, otherwise it will spam confuse ray;
-After all the eeveelutions are dead, you should have 4 or 5 team members with you. Kill the Lillep;
-Kill tangrowth and any other pokemon that comes after;
-PROFIT