Slitherine

Is it my imagination, or are the alien hordes on medium able to run rampant over your troops? Backed up the alien aggression to low, and still have problems in some instances. Even with only producing military units and researching military tech, I can only hold my own for a few years at best.
Suggestions?

I haven't played with this morning's new patch, so I don't know if the alien aggression has been modified. In my most recent game, I set the aggression to the highest possible. This just made the hives create more aliens faster, they all turned hostile in fewer turns, and they were willing to travel further to attack me. It's a pain in the neck when those power:8 beasts and power:8 fliers come attacking your bases. Many times I've felt your same frustration. Here's how I did it in my most recent game:

-I expanded slower (which is counter-intuitively better for growth... that's a different topic though).
-I stayed on the defensive. You get a bonus for defending in a base and you heal faster. Plus since your guys aren't dying, they'll go up in rank as they win battles which will build them up to be powerful enough to take out the hives later.
-Get the device in the unit workshop that gives you "+25% power when defending" (I forgot its name).
-Use field training; the rank bonus makes a big difference. It gives +1 rank to all units in a hex, so group up before deploying it.
-Focus on getting tanks asap.
-I sometimes find it useful to have one or two units with the device that allows them to capture native life. I wait to use it until one of those power:8 beasts attacks a base and kills one of my units. Sure, I lost a guy but now I can take advantage of the fact that the beast is wounded and strike back with a different unit and capture him (you can only capture them after wounding them almost till they die). This is a really nice chess move - you lose a pawn but gain a queen. Once you heal that beast back up to 100%, and use field training to max out his rank to 10, you have a pretty powerful unit early in the game. You might be temped to try this on the Leviathan or Aspidoch (enormous power:16 sea monsters)... I did this in one of my early games and was surprised to find that the upkeep was 16 credits! ouch.