This is an unmissable achievement obtained early on in the second mission.
*** Spoiler - click to reveal ***When you enter the main hall in the house with the grand staircase, go up the stairs and to the left. A hallway on your right-hand side will lead to a room with Sparda's portrait in it. This is where you obtain Arbiter. You will face your first Death Knight (possibly more than one depending on difficulty) in this room before acquiring Arbiter. Probably the best way to kill them without yet having a heavy weapon is to wait for them to hold their arm back before they're about to strike, hold 
to use Hightime, then Aerial Rave (
,,,) on them in the air. While in the air they remain prone and cannot block and/or counter.
In my opinion the best ability for Arbiter is the Trinity Smash. I would upgrade it as soon as possible. Hold
and press
,
and pause for a moment. When you pause you will see a gleam going down the blade of Arbiter, this is when you press
for the third time (still holding
) for the final attack of trinity smash. It does massive damage and gives a great boost to style rating. One execution of Trinity smash usually destroys the shield of a Death Knight and leaves them open to attack from any weapon. However, it takes a bit of time to execute, so when facing a larger group of enemies I usually go off to the side away from them and use the ophion demon pull to bring one enemy over to buy some time to execute it properly without having to stop after the second attack to evade.