Although it sucks that Jak has been nerfed more then buffed, his reduced wave concussor makes the fully charged facing away explosion that much easier to follow your Level 1 up with. You can be on the ground for most players except for the odder shaped characters like Rachet or Fat Princess. So I'd safely rely on releasing it just before you touch the ground then immediately when hitting the ground blasting off R2. The flatter the ground the better, don't let them hit the ceiling, do it as close to them as possible, you know the drill. It's new to me, I've tried it on all the characters. Some during combat others just standing. Let me know if there are any flaws, right now jak is kind of a science project to me because of all the nerfs and I don't really know how to feel about him.
P.S. - It's not a walk in the park to do against good CPU's or players so I would not prioritize it, but if you can fit it in when the timing is right it's quite satisfying. It's a skill in its own unlike "cough cough" Hadouken! "cough cough" FALCON PUNCH!
Hope this helps!