Okay, well, save your game now, because this last bomb puzzle IS timed, and the time is tight. In fact, you might want to play this part of the game in a window so you can quickly click outside the window and freeze the countdown long enough to plan your moves.
And as always, read on to the end of this hint, and I'll tell you exactly what moves to make to get out of there alive.
The general goal is to click on the large numbered tiles in order -- 1, then 2, then 3, then 4.
But you can't just click on the numbered tiles; you have to click on two small square tiles, then a medium rectangular tile, and then the numbered tile. Each tile needs to be touching the next one in the sequence along one side (not the corners), and you always have to follow that order: small, small, medium, large.
However, the small-small-medium-large sequence starts over after you click on the numbered tile. That is, after you click on the large tile in a sequence, the next small one doesn't have to be touching anything but the next small one.
You'll have three screens to do, and they get more complicated as you go along. The first one isn't too hard, but after that it's important to plan out your moves, since the small and medium tiles may lead to more than one large.
If you're having difficulty planning this out, notice that since the sequence always ends with medium-large, it has to involve a medium that's touching the large one. So look for any numbered squares that only have one medium touching them, and that's the medium that has to go with that large.
Happily, the amount of time you get is more with the more complex screens; unhappily, it's still a matter of a minute or two only. (The first screen gets a minute, the second two minutes, and the third only 2 minutes 40 seconds.)
It's good not to dawdle, but it's better to make the right moves the first time. If you make a mistake, the board boops and beeps and resets, but the clock doesn't stop; you then have to start all over with even less time. So plan it out and click once.
And of course, there's always Second Chance. What a shame we don't get them in real life!
Okay, as promised, here's how I solved the screens -- but any way that gets you there is the right one: