The pilot chose a caterpillar base for his AC so as to optimize its AP and offensive strength. His versatility in combat makes him a dangerous foe.
MK-9999 is very heavily armoured and also carries a wide range of weapons. The AC's only weakness is it is very slow, which allows even other heavyweights to outturn it and evade it's fire. The unit is a tank-leg AC that is equipped with back booster extensions, a plasma cannon double back weapon, a bazooka, a laser blade and an overboost core. Due to the Human PLUS enhancements of the pilot the unit can fire blade waves and is also capable of ignoring weight restrictions.
This guy is very dangerous with his high amount of damage. Ultimately this is a fight where you either avoid most of his damage or you die, quickly. You will be best served fighting in an open arena with either a lot of cover or in an wide open area with quad legs. You have two option here; either try and avoid his 40 or so shots (they can deal massive damage) or get inside him and blow him apart.
The best way you can deal with him is with a weapon that can deal massive damage like a Grenade Launcher or a Laser Cannon and do your best to get close to him. If you can get inside of him, you can easily out ciricle him. This means you can hit him with impunity but also avoid many of his attacks which is vital for taking him out alive.