[Agema]

I haven't got the book handy to make this precise but I believe it works a bit like this.

Your "army" for the assassination consists of two units, the commander and the bodyguard. Unit morale checks are made after a certain proportion of HP damage (20-25%?) is suffered by the unit: with 5 bodyguards (assuming the same HP per trooper), one death and another injured could trigger it. For a unit with morale abut 10, it's relatively easy to fail a morale test (not far below 50% chance), and it will carry on taking tests, with increased chance of failure, as more damage is suffered. Once the bodyguard routs, the army will have probably reached the 75% threshold for autorouting, and the commander will flee as well. So your commander never actually takes a morale test himself - the autorout accounts for him. Consequently, you'd want bodyguards with as high morale as possible. Morale 10 bodyguards are risky, 14 is pretty safe, and 30/50/99 ideal.

Because of this, bodyguards can be a liability and if your unit is tough enough, it can be best without any bodyguard. Weak mages often just need a bodyguard to hold off the assassin long enough for the mage to fire a suitable spell or three. Alternatively, maybe try a 2-man bodyguard: enough to hold off an assassin for a turn or three but not enough to autorout the commander when they go. In my experience, a quality bodyguard + commander/mage should beat a recruitable assassin unless the assassin is very well equipped with magic gear, and even beat the angel from the spell Manifestation.

Earth Attack is the biggest problem - they can even kill Bane Lord thugs. You'd need a bodyguard composed of (as per the above comment) like Crushers that the elemental effectively can't kill. The other tactic is to put a huge swarm of unimportant chaff characters with the army: spending a lot of gems and the effort of an E5 mage to kill a 20-gold scout will give your opponent second thoughts. I'd argue even your average recruit-anywhere mages are less valuable than the cost of Earth Attack, and save the expensive bodyguards for the important/rare/expensive mages.

I do have a lot of sympathies: the assassination mechanics excite a lot of frustration for many users, not just for the defending target, but the ease with which many assassins can be defeated. As Llamabeast said, it's very doubtful they are going to change, so hopefully we can just explain how it's happening and supply some ideas so you can get the best defence up.