Ni, there is no other process i would rely on, as much. There is no mind as calmed and focused as an Ni mind which is pre-aware of what might happen, specially in out of context battles. If you don't intuitively understand the essence of fight, then you will probably lose, being trapped in the over-analysis of the battle and battlefield, knowing somehow there is no real constant while fighting. I'm thinking Se and Ti might be good functions to train for combat, but Ni is the one which will be active during the fight.
I'm thinking Ni would prepare for a fight developing evasion, and precision as the result of it's capacity to quickly and suddenly conceive future possible events. It will study the body and terrain of both itself and the adversary.
Ni might as well not use brute force but direct it's speed and concentration towards sensible/lethal points to end the fight quickly.
The other thing about Ni is, it is very difficult for it to be fooled, since the user would have to fool itself, or it's internal master and mystical matrix would have to understood by the adversary to be somehow violated, which isn't even evident for the Ni itself; so it's hard to lead it to fall into a trap.
Against Ne.- Knowing how to use the terrain is a very useful ability. This creativity to where to go, how to move, in order to gather different attacking options, makes the Ne a great Offensive player.
As a defensive one it is merely weak, because the Ne cannot creatively wait hidden in the perfect place for the Ni to attack, since Ni understands the world similar to Ne and will be very cautious moving. At some point provoking Ne to fall into its' own trap. Ne looks for options and is continuously finding alternative plans. Ni is concentrated in winning the fight itself and not in the possibilities of how to win it. the framework of Ni, gives it what it need when it need it.
Against Se.- Se means hardcore developed senses and might be even better at using the terrain in an intense manner. Se would fight much more instinctively blowing fierce punches taking chances as they appear and might be much more direct and hasty. It is also a great offensive player, but this kind of adrenaline rush if weak for defense. A merely not obvious strategy will be more then enough to fool Se into follow it's steps to instinctively react and lose position of advantage. For example Se would never lose an attempt of attacking from the back; It is intelligent and not so well seen, still very effective.
Se will be winning, not, if Se is to hasten against Ni, because waiting for a resilient moment and merging with their obvious approach is easy for the Ni to follow.
Against Te.- Te might be difficult to defeat. It would be constructing creative logic interconnections while fighting and developing during combat as it understands better the way the adversary works. Then there might come an arbitrary solution to Te's mind, which is very unpredictable and intense planned.
Ni's best chance to beat Te is to direct attention to the Te's way of planning. Ni will concentrate towards the stile of fighting the Te has, and will rapidly notice Te is applying random attacks to force Ni's into following similar situations, to react and find congruence in Ni's way of working. This could become useful for the Ni, actively directing Te towards false solutions and messing with their way.
Against Fe.- Fe is empathetic, so initially it might be very difficult for Fe to force itself into a fight, then again Fe is great at defense because Fe understands how people feel at the moment and how this could develop into aggression. So Fe would rarely attack, only if compromised to a peculiar situation involving others. Their concentration then may be lethal. Then again Ni understands Fe will attack prompt and accurately leaving them carelessly easy to be knocked back.
Fe's weakness it's empathy while fighting, it won't be as deadly and will be a noble fighter. Emotionally reacting for the Fe lead it into losing focus on the target.
Against Fi.- Fi might as well sacrifice itself for the better cause. So it would fight idealistically and might be somehow pumped up by its own sense of following the right path and keep on until it is worn out.
Ni might then keep on evading intense attacks from Fi and lure them into some controversial terrain, like some place that compromises Fi to fight (like a Kindergarten ).
Fi's weakness is it's well defined sense of right and wrong, it becomes less voluble and looses chances as they come.
Against Ti .- Ti is efficient and deliberate. So it will probably have worked out an excellent location to fight, something somewhat familiar. Ti is the fighter who has done it's homework, so it will have many variables though out, sequence plans, and will be great at identifying how Ni's way is based in some malleable frame.
Ni best way of defeating Ti, is being not as congruent. Random patterns are familiar language for Ni users, so what about launching actions following the random factor of Ni. Ti can't foresee inconsistent events, and by trying this will be leaving itself into a chance factor which might as well not be infinite.
Ti's weakness is it's predictable rationality; Ti can't work if there aren't contexts to mix in order to develop a solution. Finding a pattern in Ni's behavior is most impossible, thus leaving Ti confused and disoriented.
Against Si.- Si is conservative but practical. Would make not a common attack but resist to start any offensive move until having gathered enough evidence of an easy way to attack. Si is also very ingenious, it will be concise in it's actions.
But Si also unaware telegraphs it's attacks. Ni leads Si into an apparent chance to deliver a good and easy blow, in a step by step process; Si takes the opportunity and attacks moving with caution, this makes it slower and easy to read for Ni and others.
Basically Si's geniality dies in the obvious; it will concentrate so much on experience, that it's way becomes evident for most any other function.
In conclusion Ni works as a mirror wall, that intuitively and quickly understands it's enemy, generating different solutions for each situation. Ni could fight anyone, without previous data or information, it can read the target's way subconsciously and act as it is needed during the fight.