This is a skill point.
The first level is on Planet Veldin, there isn't any elite guards on the planet yet though. The final mission brings you back to Planet Veldin and there will be tall Elite Guards which you have already encountered on previous levels, they are lanky guys with jetpacks that have powerful guns. The easiest way to get this trophy is to simply run at them and hit to jump then to do an overhead smash. Two of these attacks will kill an Elite Guard. Do this on every elite enemy you meet and you will get this trophy and skill point easily. They is way more than 10 of them in this level.
This trophy/skill point is cumulative, so if you die after killing 8 you will only have to kill another 2 to get the trophy.
Trophy + skill point will unlock upon killing the 10th elite guard with a wrench.