Marinate the filets in 2 cups of beer, salt and pepper. Let sit for 15 minutes.
Meanwhile, toast the guajillo chili for 1-2 minutes in a dry frying pan until pliable.
In a small pot, boil the chili in 2 cups of water until soft. Reserve ¼ cup of the chili water.
Place the chili and ¼ cup of chili water in a blender with the tomatoes, onions, garlic, ½ cup beer, and cilantro. Blend until smooth.
Pour into a frying pan and simmer until reduced by half. Season to taste.
Remove the fish from the beer marinade and pat dry.
In a nonstick frying pan, heat 2 tablespoons oil over medium-high heat. Sear the fish, 1-2 minutes per side.
Serve the fish immediately with the chili sauce.