MMA fight :
1. Time their initial rush/shot with a powerful rear uppercut. Then throw multiple shots to the solar plexus (this is perfectly legal in MMA).
2. Work on slipping punches, head movement, and working from inferior positions. Despite what people say about HW's gassing faster I personally want to end the fight quickly before they can re-act or even have a slight chance to counter.
3. Leg trips or worst case scenario an inside leg trip. Put them on their back. Shooting wouldn't be a good idea unless you have extremely power legs (I do but for the sake of the argument you don't :icon_lol

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4. Elbow solar plexus , and keep a good base. Go for the heel hook with full intention of breaking the heel. Most MMA fighters do not know how to defend a proper leg lock.
Or just try your best to armdrag and take the back Marcelo style. And/or rolling leg locks like Imanari does :icon_twis
Street Fight:
1. I can not stress enough how important good head movement and the ability to slip a punch is. For the most part your enemy will throw punches probably a right hand.
2. Two options keep your distance and pick him apart. By pick him apart I mean kick him in the jewels, or try to stop kick him in the knee (this might break his knee cap). Or Put the aggression on him slip and move forward go to Town to the solar plexus/body. Follow by an overhand to the top.
3. Most likely he will rush you once again you can probably time this and land a devastating uppercut or a left hook .
4. Run away as fast and far as possible. Always assume he has friends and/or a weapon.
5. If you are sure he does not have any friends clinch with him this will prevent him from drawing any weapons. Proceed to knee him in the jewels as many times as possibly while trying to obtain double underhooks.
6. Take him down , dominate him on the ground. Go for the choke you can choke people with a t-shirt ya know.
7. From here I strongly suggest running away but it's up to you. Do you want to be there and get in trouble with the police ? If you break his arm/leg will you get over it?
8. A drunk could kill you, or maybe a group of thugs did a little too much stomping to your body while you lay there motionless.
I treat any confrontation in the street as a possibility of death. It's best to keep this in mind because people do stupid things and we live in a very disgusting world.
In self defense/on the street it's best to always have the upper hand. Weight classes exist for a reason like you said and the same applies to weapons. And used your common sense most of the time fights can be avoided. Don't forget you can always run.
Whew that was long.