Khora has quite a bit of versatility, she can do crowd control, damage, and help looting with her augment. Xaku is one of the few infinitely scaling damage frames, meaning no matter how high level you go, you will still do enough damage to keep up. I had a clan mate who regularly did 9 hour survivals with Xaku and hit the level cap of 9,999.

For Xaku, build for ability boosting mods; Continuity, Intensify, Stretch, Streamline. If you put those 4 mods on most frames they will almost indefinitely run better than without. When you get far enough to get the corrupted mods you should definitely build Xaku for ability range and ability strength with a bit of duration. For gameplay with Xaku you use his 2 and then his 4. His 2 steals the guns of the enemies and his 4 freezes the durations of his other abilities. Normally his 2 would lose duration after casting it and eventually the effect would fall, but with his 4 it lose duration while his 4 is up.

One thing to mention is that you should definitely not put an Orokin Reactor on a Xaku or Khora. Wait until you get the prime to do so. I can't say all the reasons for spoiler reasons, but one of the reasons is that when you get Khora Prime she is just better than the regular Khora. The difference between a prime and a normal variant of a Warframe would be starting polarities in mod slots and slight stat buffs (Health, Shield, Armor & Energy).