If you have a spell that will knock over an enemy (or cause it to fall), can you combine it with Staggering Fall to make their tumble very mean?
Here's why I wonder: does the timing work? Any spell effect happens at the END of casting, right? If so you'll cast MM, be done casting as the missiles hit their targets, then you swift/immediate cast Staggering Fall as the target is going down and there is no issue with this combination.
However, if targets are getting hit and falling mid-cast then you can't add on Staggering Fall, because you'd be casting that spell in the middle of casting MM itself.
I feel like this is an edge case where we won't have any rules text itself, but I bet someone here can get us real close to what the official intended ruling should be.
I'd love to hear it.
If it's your turn, you can cast both Toppling MM and Staggering Fall in the same round. You would cast a spell as a Standard Action (Toppling MM) and then when they get Tripped (provided the target doesn't have a Shield spell), you can use your Immediate Action in your same turn to cast Staggering Fall. Likewise, if you have a group member who is adept at Tripping, and they trip an enemy on their turn, you can use your Immediate Action to cast Staggering Fall as well (provided you have an IA available for that round). And even if it's the enemy's turn, and they do something that causes them to fall, you can use your IA to cast it even though it's not your turn.
The timing of using an Immediate Action is that it happens anytime you want it to, at the exact instant that you want it to, and even when it's not your turn. The only limitations to being able to use an IA is that you cannot perform both a Swift and an IA in the same round, nor can you perform two IA's in a round, and the only thing that gets resolved sooner than an IA is an Attack of Opportunity.
So for example, lets say your ally has Greater Trip, which provokes an AoO while the enemy is falling prone, and let's also say that you have your IA available for this round. Your ally successfully trips a target, and you say "OoOOO! GM! I want to use my IA to cast Staggering Fall!", your GM would have to allow your Ally to first resolve his AoO (because all AoO's are resolved immediately at the exact instant of being provoked), and THEN you could use your IA to cast Staggering Fall. So there is an order of precedence when using your IA to cast Staggering Fall, and it only comes behind AoO's, but for actual In-Game Time purposes, your ally's AoO and your Immediate Action would theoretically happen within ~0.01 seconds of each other.
I said round, but I meant turn. Here's the rule I'm talking about.
Immediate Actions
Much like a swift action, an immediate action consumes a very small amount of time but represents a larger expenditure of effort and energy than a free action. However, unlike a swift action, an immediate action can be performed at any time—even if it’s not your turn. Casting feather fall is an immediate action, since the spell can be cast at any time.
Using an immediate action on your turn is the same as using a swift action and counts as your swift action for that turn. You cannot use another immediate action or a swift action until after your next turn if you have used an immediate action when it is not currently your turn (effectively, using an immediate action before your turn is equivalent to using your swift action for the coming turn). You also cannot use an immediate action if you are flat-footed.