In an MMA sport context, I would jokingly say: lose weight so that you are not in his division.
I am 5'10." I work with a lot of people who are 6'2" and above. I have sparred with people who are 6'8." One tactic that I have found to work reliably well is to work the low game. Work on staying several inches lower than one normally would and working a fast bob. Narrow the ranges that the blows can come from, forcing the taller person to punch down. This takes some of the sting out, unless they are very good at sledging energy and also forces them to open their head up for an overhand, every time they throw a punch. Attack the body and legs. If viable, the groin is at very effective punching height, even for uppercuts and body hooks.
I liked the mind-hit theory of when the taller person punches, kick. And when they kick, go inside and punch. Never tried it specifically but I like the concept. Other things that need to be practiced heavily, if you are going to stay low are a decent shoot and a dynamite fireman's carry or wheel. Sooner or later, the taller person usually decides to 'pancake' and the shorter, low fighter needs to be ready for it. The counter is to dump them and scramble for the back.
Problems with fighting low are that it can limit mobility if one is not used to it. And it also burns out the legs almost as fast as the JKD shuffle.
With weapons and super-extreme reach, the main thing is to stay off the thrust line as much as possible. And remember the fulcrum mechanics. The longer the lever, the wider the swing arc and more distance it has to travel. Try to force the swing and play in the greater time gap.