This solution is for the mission MAMMAS BOY. To get the 100% you must win the fight with no more than 10% loss of health. I was having a hard time with this mission and found another way. What you need to know is there is a hospital 2 blocks down from the LC Cage Fights, around the back on the wall behind some ambulances is a health kit.
Ok here we go, choose the mission MAMMAS BOY from the cell. Go into the fights and do the best job you can to win. You will be placed back outside with the instruction to kill Santos,now run away from him to the first aid kit, grab it turn and kill Santos. Now you will have finished the mission with 100% health easily getting the mission done.
The above solution assumes you have already beaten the story mission and time requirements. If you haven't then you must hustle to beat the 3 minutes and 10 seconds time limit.
One more tip id like to share, several missions spawn you without a vehicle or in a shit one. When starting these missions go to where the mission starts now call Henriqe to bring you the sports car of your choice wait untill he is almost there and chose the mission from the cell phone. You will find that you can easily grab the car now and complete the mission. I found this very helpfull on SEXY TIME and several others.
This above tip assumes that you have completed the story and are accessing missions through the cell phone.