Prepare gas or charcoal grill, or preheat broiler.
In shallow dish, combine chicken, shallots, lime juice, garlic, cilantro and black pepper; toss gently. Allow chicken to marinate in refrigerator for 2 hours, or overnight.
Make salsa by combining in medium bowl tomatillos, tomatoes, red onion, jalapeno, lime juice, garlic, cilantro, oregano, cumin and salt. Store, covered, in refrigerator.
Place chicken on grill or under broiler and cook, about 5 minutes per side, or until done throughout. Remove from heat and cool slightly. Slice chicken on bias into strips. While chicken is grilling, warm tortillas by placing 1 at a time in medium or large frying pan over medium-low heat.
To serve, spread each tortilla with guacamole, then top with several strips or pieces of chicken and a tablespoon of salsa. Roll into a cone shape and serve immediately.