You know a bit about arrays ? If not we have good Tutorials for it ;)
Make an Array and save there the pressed keys like: Array(1: Z, 2:Y, 3:Y, ....)
In you On key Press function something like:
+ On Key press
++ If Key = Z
+++ if Array = Z, Y, Z
----> Special skill Haduken
----> reset Array (optional)
+++ if Array = Z, Y, Y, Y
----> Special Skill Sharingan
----> reset Array (optional)
This is just an idea, try it on your own risk :D