Easiest way? Open GAMECONSTANTS.XML.
About halfway through the file, you'll see a really obvious list of damage multipliers for each unit's weapon versus each unit's armor. The space units are first, followed by the ground units. The numbers are multipliers, so the higher the number, the higher the damage. Upping them makes things die fast.
Almost all infantry use the same damage type (I think the infiltrators are the only ones who don't.), so it's easy to edit just them.