This can be obtained on the very first battle of the game.
The easiest way to build your combo is to use beat downs.
Lead off with
which is the Cape Stun, then quickly mash
until the enemy dies, then rinse and repeat on other enemies. Use
to counter-attack enemies that try and hit you (you'll know when they are going to hit you, they will get blueish lines above them if playing on Normal Difficulty).
You're combo will break if you get hit, if you take to long, or if you use
by itself and miss an enemy.
However, your combo stays intact for a relatively long time so as long as you don't get hit by an enemy you should be fine, also if you use the Cape Stun and miss an enemy it doesn't break your combo for some reason.
These general tips can be used to get the x25, x50, and x100 combo as well (albeit the x100 can only be obtained in one specific fight).