[Agema]

Assassinating anyone just halfway important, historically, was very difficult. For every successful assassination recorded, I'd guess there will be at least one recorded unsuccessful one, and add in all the unsuccessful ones never recorded.

Assassins are also relatively cheap. If they are made better, they'll need an appropriate cost increase. You can of course make assassins better with gem expenditure: I'd reckon for 20 gems or fewer and Construction 4 you could equip one with a high chance of killing most adversaries (including mages) and even ones with a bodyguard of basic troops. Indeed, a bodyguard might make some easier to kill because they may rout and take the target hero with them.