Fighting clean is asking to get the shite knocked out of you, if its not a competitive martial arts type fight, where both participants agree to spar.
Good techniques that require relatively little strength:
Open palm strike to the solar plexus. Or pivot on the back foot and ram one's elbow into the same. Elbows and knees can deliver a lot of force, and regardless of the size and strength of the attacker, there is no overriding the effects of a blow to pressure points.
Other convenient sites include the brachial plexus on the back of the neck/shoulder,
Common peroneal nerve, running on the outside of the lower leg. Strike with a kick or object.
The golgi tendon (elbow), if struck hard with a back-fisted strike, knuckles etc. will force a relaxation of the joint, allowing the joint to be wrenched apart and broken with minimal force.
Typical starting technique for your average meathead thug is often a hook punch, respond by either moving in close, blocking the blow with a forearm (better, blocking with a fist driven into the biceps, or blow to solar plexus, knee into groin, pull down kecks, grab their nadgers in hand, twist and pull hard.
Another good one is to step past, ducking under a blow, allowing the opponent to over-reach their center of balance, before delivering a short, vicious stamping kick to the side of the knee, it will break fairly easily if hit from the side.
Stamp on the ankle from the side, or from the back, aiming for the achilles tendon.
For those strong enough, try crouching, grabbing legs in a bearhug, standing up quickly, then pulling apart legs, go down on one knee, bring target's groin down onto waiting knee, or twist target sideways and bring down stomach on knee hard.
Can do the same with the opponent's back. Although this risks breaking their spine and paralysing the opponent permanently if done hard enough.
A sideways-on elbow strike to the bridge of the nose will almost certainly break same.
