This is a fairly straight forward Achievement. You need to obtain 5 Kills with the Hammer during any match on the Mythic Map Assembly, other than a Team Swat match since the Hammer will not spawn in that type of match. These 5 kills can come at any time and DO NOT need to be in a row. One here, Two there, etc. Just five kills in the match.
The easiest way to get these kills...After obtaining the hammer...is to wait inside one of the two bases. The big advantage that the two bases provide is that while you can look through the windows to see the rest of the map, people from the outside CANNOT look in. So you can camp at the entrances and hammer unsuspecting passer bys.
This is just one method. To be honest, the 5 hammer kills can be acquired any way you want. Feel free to charge blinding into a hot zone to get a kill. Just remember that you're going to have to then get the hammer back after you respawn. So do it any which way you see fit.
And above all, just have fun with it. Other Mythic Achievements are going to be a lot harder to get than this one, so savior in your moment.