The ultimate purpose of Lileep/Cradily is to be a special tank, due to its good physical attack/special defense, and decent coverage.
Here's an example set for RSE UU:
Nature: Adamant (+Atk, -SpA)
EVs: 252 HP / 164 Atk / 92 SpD
- Rock Slide
tl;dr Sponges special attacks, hits back hard, heals damage, wears down mons.