Combine aren't nearly as big a threat as the Empire. True we've only seen a glimpse of the combine on earth, but other than that they're just your usual planet hopping conquering run of the mill enemy alien faction. Their only super heavy weapon shown up until this point would be a strider.
Practical sense in mind, stormtrooper armor is made to make projectile weapons obsolete, kinetic energy weapons like rifles won't kill a stormtrooper like it would a normal human. You need a high grade energy rifle to even get by the armor, and even then the stormtroopers seen in the movies don't always die when they get shot. Their armor can disperse the energy of a blaster rifle, which will knock them out but will save their life. It works and it doesn't work, it's not a guarantee they'll survive but there's also a chance they won't die.
Haven't even thought about the empire's more secretive weapons. There are some weapons in the star wars universe that make the deathstar look like a toy sword.
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