This is an unmissable achievement obtained about halfway through the second mission.
*** Spoiler - click to reveal ***Soon after you have obtained Arbiter at Sparda's portrait, there will be another similar room, this time with Dante's mother's portrait (Eva) where you obtain Osiris.
In my opinion, a couple of the most useful combos for Osiris are Double Up and Prop and it's successive upgrades (Shredder and Shredder, Level 2). Prop is useful both to boost style rating (when fully upgraded it can boost about a full style letter) and also does a decent amount of damage, useful for killing Frost Knights. Also, prop is very useful for destroying the shield barrier witches put up, just be careful when up close to watch for the swords they shoot. Double Up is very useful when facing multiple stygians as well as a Tyrant or a Butcher, because it boosts you far enough into the air to damage the stygians while staying out of reach of the Tyrant or Butcher. You usually want to use the angelic weapons (Osiris and Aquila) for larger groups of small enemies, so I find the charge attacks for them aren't very useful because you don't always have a lot of time to charge them.