It boils down to basic formulas...
Defense is dependent on Psi Str + 1/5*Psi Skill
Attack is dependent on Psi Str * Psi Skill
So the defense raises in a linear fashion, while attack raises exponentially. That's why you have to be very careful when assigning such values (and also why 100/100 soldiers/ethereals are absolute monsters in attack but wimps in defense against each other).
Maybe you just were doing it wrong, Harald. Instead of taking any chances of getting MC'ed, go full aggressive, scout with tanks as much as you can and MC every Ethereal, giving them no chance to retaliate with their own attacks (as they have never seen any of you soldiers)
Not saying that such a mission is much fun, either - but at least you'd have to enact a radical, unusual tactic