This medal to start with was ridiculous to try and achieve, in order to get just the bronze you had to get 75 nemeses kills.

In order to acquire a nemeses you need to be killed by the same person 5 times without you killing them in the same match, when you have a nemeses you then need to kill them with an execution. This was revised in the latest update (Thankfully) to make it much more manageable. Still, having this come naturally is still rare, for me anyway.

Here is a way you can get this onyx reasonably quick and get this medal out the way.

I play in a private King of the hill match on checkout with me and a 2^nd controller, No bots.  In the weapon set up you want to replace the incendiary grenades and the ink grenades for the longshot. With player 2 you want to go to the platform were the longshot will now be, this puts you high up and enables you to see all round the level, pick up the longshot and kill P1 with a head shot 5 times so you are now a nemeses to player 1.

Be sure to wait a few seconds after P1 Respawns as they will have Respawn protection on. Pop the P2 controller down and pick up P1 controller run over and execute P2 and this will get you 1 point towards your medal. Rinse and repeat.

You can also pick up some Frag grenades as P2 or just use your smokes and plant them someplace so when you play as P1 and run over to execute P2 you can shoot his grenades this will give you points towards your Sapper Medal also.

Thanks again, hope this helps

Stimp313