Keep moving constantly till your opponent begins to slow. At that point you can use your speed to slip into close range & unleash some punishing offence of your own.
Use the element of surprise. If you suspect things are about to go nasty, be the one to do the 1st move. End a tense standing moment w/ a sudden, well-placed blow to the jaw or solar plexus. Put everything you've got into the strike - try to put a stop to the fight in a single manoeuvre.
Stick & move. Take your time staying away from his strikes, waiting for an opening & then closing the gap & landing a hard shot. Attack the body when you don't have a clear shot at an opponent's head. When attacking the body aim for either the ribs or the solar plexus.
An open hand slap to the ear is just as good as a left hook for breaking an opponent's equilibrium, if not better.
Hitting their nose can blind them momentarily by causing their eyes to produce tears, which gives you an opportunity to deliver a fight-ending punch.
A kick to the groin will compromise their mobility.
If you get taken down w/ your opponent on top of you, & you're unable to get onto your feet. In this situation, scramble for a hold or lock that incapacitates an attacker by neutralising a single part of the body. By applying an armbar, choke, or joint lock, you may be able to do enough damage to end the scuffle. Apply a rear naked choke or triangle choke, depending on your positioning.
Bend fingers back, bite, chop the throat, gouge eyes, pull hair, & do anything else you have to do to survive.
Guns, spears, & swords are good against a big guy.
Wearing a mask would help.