Well, if your Alakazam is Lv. 100, it doesn't need the most powerful move to OHKO opposing Pokemon. I recommend this moveset:
Alakazam @ Leppa Berry
EVs: 252 HP, 128 Def, 128 SpD
Nature: Timid / Modest / Adamant / Jolly
-Confusion / Thief
-Psycho Cut / Dazzling Gleam
Confusion has 25 base PP. This is one of Alakazam's highest options. Thief has equal PP and hits dark types, however. Grass Knot and Psycho Cut have 20 base PP, so they are good options as well. Dazzling Gleam, however can be good if you are battling a high level dark Pokemon and Grass Knot won't OHKO. Recover is Alakazam's highest PP recovery move. Leppa Berry is a better item choice in my opinion, because Alakazam can already recover reliably using Recover, and the 10 extra PP is very useful. Finally, I recommend a defensive EV spread, as Alakazam appreciates the extra bulk for more longevity, and he is probably already going to outspeed and OHKO everything without the EVs.
Good luck. :)