i had a little trouble with the bair spike so i didnt really feel like it... but the dair spike i couldnt do either. so i went to training against a lv 9 fox and put him at 100% then forward tilted, ran straight off the stage, timed my second jump and either caught him in it or planned ahead for his upb. just a little tip if you need help with it. also it WILL kill them at this percentage too. the dair hits get him to about 131. the bair spike is at the second hit basically, and it sends you up, so it's good, but situational like snakes.