Depends on assumptions of technology used. Things like lasers can be
iffy, as they are not necessarily very effective weapons, and other
particle weapons may have issues in an atmosphere. On the other hand,
things like real anti-gravity means that "they" can do a lot more than
fly fast or radically maneuver, things like tweak time/space reality or
the nature of local universal forces, so that "they" wouldn't need
weapons in the normal sense at all. Which also begs a more basic point,
what would a really advanced technology/society need with bothering with
Earth?? There is nothing here that can't be gotten/made cheaper closer
to "home". Like the gods of old, we are projecting our own fears or
wishes into these "aliens", giving them motives and technological
assumptions that fit what we believe. If the real thing ever shows up,
they'll be more different than what we might expect.
But enough of this rational wet blanket, basically Earth is screwed, as
even fairly conservative Sci-Fi aliens can whip our butts (barring some
ridiculous secret weakness that will bring us final triumph over the
icky green hoards).