Excluding side character levels, this is your third visit to the Valley of Seers, this time as Ganondorf. Since Lana has the Triforce of Power, he will go to no end to get it, but has to deal with many challenges to do so.
Ganondorf to the rescue
Ghirahim will be trapped at the southwest base while Zant will be trapped at the northwestern base. Fortunately, there's a way to get to them in time: if the north part of both bases are open, they link like a maze; just skip through the enemies until you reach either of them. On the 3DS and Switch, you can choose to play either of them once they've been rescued.
Trouble with Impa
Things will get harder now that Impa has come. She'll create multiple copies of herself, making it hard to tell which one is the real Impa. Depending on which version your playing, even in co-op, eliminate all of the Impas, as they'll cause you trouble otherwise (even at your Allied Base).
Showdown with Lana
Before facing Lana, make sure you take care of checkpoint officers and take all bases to avoid dealing with endless enemies coming at you. Most importantly, get Ghirahim and Zant, waiting by the Fairy Fountain in the middle. You'll soon find out why, as there is another surprise behind this. Go to the middle of the north part of the map, where the enemy base is. Deal as much damage to Lana as you can, and as soon as you finish, run to the Fairy Fountain.
Triple Lana Trouble
Impa has prepared three magic circles for three Lanas to escape, but don't panic! Ghirahim and Zant should automatically chase one of them. If you manage to get there early enough, you should be able to attack at least two to three Lanas while the third one is slowly escaping. It's all a matter of time, so attack two quickly and run to the third, then attack before the last Lana can escape. It might take numerous tries, but it's worth the challenge to see Ganondorf run for vengeance!