You can mime limit breaks with the character who first performed it, but if you try it with anyone else, it won't work. And of course 8 Mimes and 8 Counters would make you very strong, but who can be bothered to master both Mime and Counter 7 times each? By that time, you're probably on level 99 already, so you'll breeze through everything anyway. It's also quite rare for Omnislash to do 9999 damage with each hit. However, if you really want to cause the highest damage ever, use Cid's Highwind limit break and pump it up by using W-Item and Hero Drink about 5-10+ times on him because I think Highwind fires its missiles at least 20-25 times or so. Omnislash hits "only" 15 times.