I used a 2 hoplite guard on machaka's mages against the ai. 2 hoplites will beat any standard assassin...and the ai had been killing my mages. Also I think that 2-3 will kill any standard assassin, and if you're facing a particularly customized assassin that can kill 3 units it can probably kill 5 units, unless you feel like using hunter spiders or niefel giants as bodyguards.
I often even just assign one elite infantry to mages so that they have some chance against unequipped assassins.
Also in combat I've decided on the flank and back or just back, not near archers ever as the best mage placement. Back so arrows don't hit and not directly behind troops or near archers so attack rear and attack archers don't work. Of course they can still be flanked so keep something on your flanks.