Whiscash Issues - Using ItWhiscash has problems getting in on a sweeper. Even then, you have to get the Substitutes up before losing too much HP to use them. Grass Knot, while weak at Base 40 VS 'cash, is still a quad weakness, so you may inevitably run into no-Substitute issues or a Sp.Atk high enough to 1KO (with or without buffs). The issue is knowing what will switch into you and what to switch out to preemptively.
Whiscash Issues - Fighting ItGetting into 'cash safely is roughly as difficult as getting it in safely yourself. A free Dragon Dance on a switch-in will really bite you in the *** when it follows with EQ, Stone Edge, or worse. A free Sub will do likewise, protecting it from your attempted Wall. The issue is knowing what 'cash you'll switch into and what is backing it up.
Using and fighting Whiscash is effectively a chess-match: Know where and when to move and punish mistakes without making them. Notable multi partners include Flying Types, Water Absorb, Storm Drain, and Levitate Ability Pokemon, and (in any case) Sap Sipper Ability Pokemon as backup.
Sap Sipper Azumarill is a great back-and-forth (switch repeatedly) partner. It relies on luck and the psychology of your opponent to work, but you'll wear down Grass Move PP if done properly, protecting Whiscash. Using a third Move Type will probably risk a third Pokemon being immune (or worse) to the Move (such as switching to a Fighting Move and a Ghost Type appears or a Psychic Move into a Dark Type, like Zoroark).
On the other hand, getting a solid hit into every opponent with spectacular prediction will ruin Whiscash and every of its allies. Being unorthodox in your offense and defense makes Whiscash that much less predictable.