Which ones are 24 and 19? I'm not near my copy of the game and don't know them off the top of my head. As for comboing into instant kill, it tells you the basic parameters before the trial, but doesn't really tell you what the parameters do to LET you combo into instant kill.
Essentially, when you meet the requirements, going into instant kill mode causes a sort of freeze, kind of like when an overdrive starts up. Whereas normally the game continues around you, leaving you open to attack or at the very least allowing your opponent to recover from whatever you've hit then with.
You SHOULD be able to do it after a wall dust in the corner, the only issue is finding a combo into dust for your character. They make it fairly easy though since they give you a full meter. I found a combo with Ky that was simple, did enough damage to get Pot into 10%, and used a red RC into wall follow up, leaving me with 50% tension, meeting all the requirements. The actual combo into IK at the end was just a matter of not dashing to much and getting the IK to come out as early as possible after the wall stick.
Other tips I can think of are use training mode to practice the combo before hand, you can set up Potemkin as your dummy and set his life to 21% (I believe that's what they have it set to in the mission). You can also use training mode to see firsthand the changes in IK, set opponent to 10% life, give yourself at least 50% meter, activate IK, and you'll notice the freeze and that your aura is gold rather than red.