I'm not sure that beating the drow would be that easy IO. Yes they have what you say, high level wizards, numbers and a united army. Thing is the drow have that and more on their side. Each drow, even the least, has a pretty good resistance to magic and a great deal of superior equipment (granted it can't take daylight but most drow don't run around in that to begin with). The more experienced drow of course, have even better toys. They also possess a great number of high level casters, both mage and clerical, of their own with a huge slave population ranging from kobolds to minotaurs and greater which they can toss around as well as exotic units like lizard cavalry which are unimpeded by most terrain. And if the King of Tribe's comment on elf-girl children was heard by most female drow, as they worship Lolth, it would have them enraged to an almost unbelievable degree.
The combined drow-slave population
of most drow cities is outnumbered several times over by the fighting strength
of the Khaduli horde. The same weight of numbers means the drow casters are badly outnumbered by their Khaduli equivalents. This is the army that routinely mantains hundreds of bound demons as cannon fodder and summon great elementals as a back up plan. And again, the division and lack of trust in the drow ranks leaves them at a crippling disadvantage. And that's without actual treachery happening and when one remembers that the great houses of Menzoberranzen turned on each other during a battle to resist an invader who intended to annihilate them, treachery is likely to happen.
If the drow had to fight the Tribes on the surface, it would only be at night which would be to their best advantage as even surface elves have a hard time at best detecting drow at night. The Tribes would have their sentries shot by drow crossbows and put to sleep (via the sleeping poisons drow commonly use), probably have their necks slit as the drow pass by, and the morning after would be counting their dead in the thousands as the drow would murder them while they slept. If detected while doing their dirty work, they would flee into the night while the humans stumble around with lit torches which would let the drow know exactly where they are and make them easy prey for ambush if they didn't just avoid them completely. Not even wood elves, arguably the best trackers on the surface would want to try and chase drow in the night. Add in that the drow would never have an above ground camp, using caves which probably would need magic (such as a portal) to access, the Tribes would have a hell of a time at best assaulting their forward bases, which could easily have the tunnels to the surface collapsed if they could be found at all.
I have no idea why you assume that all the fighting will occur at night. The drow are at a significant disadvantage during daylight. Guerrilla tactics will work, to a limited degree, that's why Nalifan has unleashed rangers. The Khaduli, on the other hand, can lose ten thousand fighting men without significant degradation of their combat effectiveness. The drow can't say the same.
Underground it is even worse, as the few places there are that are large enough for a horse charge tend to have bad ground for such, while the drow lizard cavalry have no troubles with wall and ceiling climbing. Considering the basic factor that the Tribes (as being human) need some light to see and fight by, which the drow themselves do not need, is easily countered by the drow's innate darkness magic which even the least of drow can use. Add in drow's innate invisibility combined with their piwafwi (drow cloaks) and you can have ambush central anywhere the Tribes are even in the midst of their formations (such as levitating down from the stalactites) while they stumble around blindly in the dark. If the Tribes went into the Underdark to fight the drow, they would (IMO) be wiped out, either slaughtered by the drow in combat or taken captive and either sacrificed or pressed into slavery.
This assumes the Khaduli will have very limited access to light, can't shoot lizards off ceilings, can't counter darkness magic despite having vast magical resources, and can be entirely defeated by guerrilla skirmishes. Most tunnel attacks will be suicide because the Khaduli will just crush the attackers with numbers as they can afford to trade five to one against the drow and win (and the drow won't get those ratios) and those attacks will mostly not happen as no drow warrior thinks that killing three or five or seven Khaduli is worth his or her precious tender buttocks getting stuck in a mob of human killers. They're drow, they don't do self sacrifice. That's for faerie. Again, the drow cannot withstand the Khaduli and are likely to badly underestimate them.
Ultimately the Tribes are great in open combat on the surface particularly on horseback, but have no skill or knowledge of tunnel or dark fighting, which is something the drow have in spades having done it for thousands of years.
The Khaduli have no experience in siege warfare either but have cracked every fortress they've faced in less than a day. Their elite is fueled by black magic, their leadership is a ruthless meritocracy, they have vast numbers, they have enormous amounts of brute magical force, and a powerful epic wizard isn't their heaviest hitter. This is an army no one sane wishes to face.
The Excellent Prismatic Spray. For when you absolutely, positively must kill a motherfucker. Accept no substitutions. Contact a magician of the later Aeons for details. Some conditions may apply.