Let me recast your question in a more precise way. First off, you're missing some information: namely, what that crystal block you've put in there is supposed to do. So I'll ignore it.
1) Setup
Other than that, I've already given you all the tools to make this happen. Consider that you make twenty combos, the first eighteen of which are all copies of each other and are the floor tiles. They are all linked by combo animation to eachother in a big line, ending with the last two that show the floor tile falling.
2) Falling at Different Rates (by themselves)
To make them all fall at different rates, just place different combos from the sequence on the screen. For example, for the first tile you want to fall, place the combo only two or three combos away from the falling animtion in the combo animated chain of twenty you set up. Then place the combos in ascending order, further up the chain. You see what will happen, right? The combos you placed further down the chain will complete the combo animation first; so they'll fall first.
Bottom line- you're in control of when they fall.
3) Controlling the Combo Animation
If you dont want them to fall immediately, you can set the combos in your chain to appear as secret combos. This can be useful for some really annoying but pleasing effects... a little creativity reveals this. You could set a trigger-temp on the floor so the tiles don't start to fall till Link steps on it. You could set the undercombo as a combo-animated falling tile and then make the floor slash combos to discourage Link from using his sword. Or the floor could be made up of pound combos that dont start to animate until link gives them a good whack. Etc, etc, etc.
I re-iterate- Combo animation has AMAZING potential. Newbies, take heed. First get your mind around warps, layers, and boss flags, and all that... but then BECOME MASTERS OF COMBO ANIMATION. The potential is (so far) virtually untapped by the usual suspects of quest design, so if you wanna wow someone... use it.
As long as I'm on the topic, some more ideas for Combo Animation:
1. Imbue Link's sword. Make all the walkable tiles slash combos, and make the undercombo of the screen a copy of the regular floor; only have this copy combo animate to show some sort of magical splash, then have it animate back to the original slashable floor. What will this look like? With slash on, it will look like there are splashes of (electrcity, bubbles, whatever) coming out of Link's sword as he slashes it.
2. Alternating blocks. A simple idea, but also untapped. Simply have two sets of blocks in the room that move up and down by themselves, impeding Link's progress while up. Could make for interesting puzzles when joined with push blocks...
-C