How do you fight someone 2 times your size but kinda old?

im goin to fight my grandpa every day since i was 5 he hit me and punched me in the face and now im 15 and im going to kick his *** and he still trys to do it he was in the soviat union army but now hes 76 how do i fight him i want to kick his *** so bad that he cant get up but hes still stronger than me