The game utilizes Lock-On Technology (the second ever FNAF fangame to do so, only behind the first FNAMiku's). So here the steps to unlock Waluigi:

1. you just have to beat Night 1 of Five Nights at Wario's 2 to unlock Waluigi in the Custom Night

2. Go to Extras in FNAMiku's 2 and click on the FNAW2 icon to unlock Corrupt Tails.

3. Then boom! Waluigi is in Five Nights at Miku's 2. When he makes his way to your basement, you might wanna turn around.. or else...