Easy really, but if you don't have a guard low enough to counterkill, and don't 
want to spare the time to beat them into a kill combo just wait until they attack 
(this seems to work best with enemies that have slower weapons) and deflect (block) 
it then immediately attack as they recover. Slower enemies like Brutes will be 
vulnerable to a combo kill now (tap attack button when you hit) and easily dispatched.

Also with dual assassin blades, run up a wall without locking on and you can 
drop-assassinate up to two guards at a time easily. You can do this again and again 
potentially but the move is a little glitchy in some places. 

Additionally, with dual assassins' blades, I find that guards are often very vulnerable 
to quick assassinations if you attack quickly, before they pull their weapon and enter 
fighting stance, and when they're moving into position early in the fight. If you move 
in rapidly or drop from a wall and catch them before they can prepare for combat, you 
can kill 2 and then as they fall move in and semi-spam assassinate and often kill all 
4 guards in a group before any of them can do jack. Ditto goes for grabbing enemies 
and throwing them to the ground, then assassinating them. It's not always practical 
in combat, though.