When you meet hostile aliens, you should probably want to eliminate them
before the whole Earth is covered. First you have to be (almost) sure
it
is an alien you are going to deactivate (see "Alien Identification").
Here are some proven methods for exterminating aliens:
1) Bare fists are in most cases too weak a weapon, unless you are a Cyborg. But in emergency situations they may be the only useful way.
2) Sticks of wood, or some kind of staff, are quite good in close combat. These objects can be found in a local forest.
3) The most effective close combat weapon is a club or baseball bat. Softer aliens may perish while you smash it with a club. But if the alien is armored, you better run.
4) The best weapon that a normal human can use is some kind of hand gun. For example, a .44 Magnum is likely enough to stop most lightly-armored humanlike aliens.
5) Against groups of aliens, hand grenades are quite good; they may not kill them, but should at least injure them. Some of them may only be frightened, but then you have got some time to run away.
Eliminating is not always killing. You may capture and torture the alien,
so it may promise not to come Earth again (if it survived torturing).
If you feel too weak to eliminate an alien, try to find some group powerful
enough to do it for you (and Earth).
Maybe the best solution is to make peace with the aliens instead.