Just follow the order of buying items and skills and you´ll be fine. Crucial for this build is to co-op with finisher hero.
You place the totem to cut the way for enemy heroes to their base. This part cuts off enemy because dead spawns slow enemy and hit at one time. You and co-op partner have plenty of time to finish the job.

With this build and good co-op partner you will get first blood and continue to kill enemy till the end. Score 15kills and more is guaranteed. Of course if you get good team.