This exercise seems to be simple. We only need 64 characters and then the 0xdeadbeef value. However there is a hint that this exercise should be solved with less than 10 bytes.
The sprintf function ‘transforms’ the passed arguments into the buffer. We can exploit this feature: if we pass %064x first, then it is converted into a 64 character long buffer. Then we can pass 0xdeadbeef. The solution:
/opt/protostar/bin/format0 %064x`python -c ‘print “\xef\xbe\xad\xde”‘`