Play as a weak character that doesn't deal much damage. Sunflower variants are good at this since she doesn't deal much damage like the other plant classes. Try to avoid vanquishing opponents, just weaken them and let your allies finish the zombie off so you will receive the assist credit. If you are on the zombie side, the Scientist Zombie can be helpful if you use him at long distances since he deals very little damage compared to the other zombie classes. Try to stick with other zombies and deal damage at a distance with them to maximize assist chances.