In Call Of Duty Black Ops 111 zombies on Der Eisendrache. What are some tips for getting better?

I have played before and got to round 18. I was wondering if there was anything I was missing. Ive used perks and all the normal stuff. As a side note

I have played before and got to round 18. I was wondering if there was anything I was missing. I've used perks and all the normal stuff. As a side note, what is the easiest bow upgrade to complete. Thanks again!

or:I have played before and got to round 18. I was wondering if there was anything I was missing. I've used perks and all the normal stuff. As a side note, what is the easiest bow upgrade to complete. Thanks again!

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