Arrange oven rack 6 inches from broiler. Heat broiler on high. Working with one at a time, add the chicken breasts to a resealable plastic bag. Using a meat tenderizer or heavy rolling pin, flatten breast to an even 1/2-inch thickness. Transfer flattened breast to a plate and repeat with remaining chicken. Sprinkle the chili powder all over chicken. Season with salt and pepper.

In a 12-inch oven-safe skillet, heat the oil on medium. Cook chicken breasts 4-5 min. per side, until fully cooked. Remove from heat. Top chicken with the guacamole and cheese. Broil 1-2 min., until cheese is melted and bubbly.