What have changed here? Well, if you're observant, you might note that two command blocks are missing. If you want details...
I have rearranged the things, but I have mainly removed one scoreboard (a counter that allows you to keep track of some things), so that it reduces the already super-low lag, and changed the rest so that it works with the change. Funny enough, removing it gave me more space. So... that was some hardcoding before hand.
Basically, I did some things that took some time, but I can't really explain them to people that aren't part of the no-life-people-who-code-with-that-strange-stuff club.
I planned on changing the thing so that arrows also explodes on mob contact, but I can't make it work. Ye. Text.