This is an old favourite, which crops up about every 2 months or so. There are numerous ways to make stronger soldiers - either by having a looping script which reduces the amount of damage a unit takes, when he takes it, or by adding a one-liner 'hit' eventhandler, or by modifying the config of a basic soldier and increasing the armor, armorstructural, armorlegs/torso/etc values. If you would like elaboration please say so.