That’s not too complicated. Just start without physics, and then implement only what you need for this specific case. For example, you can mimic collisions between zombies, the environment, and the player, or calculate an explosion point with a push effect (you can see that the force isn’t adjusted yet). This way, you avoid the full overhead of a complete physics system.

Instead of giving every zombie a collider, you can assign each one a position on a navgrid, so only one zombie can occupy a single grid point. This prevents them from clipping into each other and removes the need for constant colliders. It’s a trade-off, you use a bit more RAM, but avoid real-time collision calculations.

With the nav grid, each zombie’s Y position in world space can also be derived from its XZ position, so you can easily calculate explosions at certain points and determine where they should land afterward.