First it's worth taking note that this is for not killing 5 enemies all at once. This is a cumulative achievement, meaning you can get one kill at a time during different games. You can also kill any type of enemy which includes the AI controlled Grunts and Spectres.
This achievement can be done early on, but it's recommend that you get to level 16 in order to unlock the Warpfall Transmitter Tier 2 Kit, which will allow you to drop your Titan in about 2-3 seconds, which is much faster than normal.
Next you will have to play a game of Attrition, Hardpoint or Capture the Flag, and score enough points in order to earn a Titan. Keep in mind that burn cards can be used to accelerate this process if you are going for this achievement and want to call in as many Titans in a game as possible.
Once the Titan is earned, it is recommended that you run around the map until you find a group of Grunts that aren't moving, somewhere in the open. Ideally, you’ll want to find a group that just landed in a pod as they will emerge together. Use the to drop your Titan while aimed at them. If the Titan lands directly on an enemy, you will get an “Auto-Titan Kill” show up on screen and make progress towards this achievement.
Alternatively you can try to drop your titan onto another Titan during a fight. If the enemy Titan is rodeo’d and kneeling it will be an easy hit, however enemy Pilots will still have a good chance at survival and escape even after impact and you’ll only end up with some assist points, so this method can be a little risky. Also, if you just play enough games, your bound to get 5 by sheer luck alone, so keep that in mind!