5-3, then:
The first part is actually very (well, kinda) simple, it just has to be very precise.
Grab the key immediately, throw it on the conveyor belt, and follow it under to the left. Grab the key again, then stomp the conveyor belt switch, jump on the spring, and hit the yellow switch.
Throw the key on the second conveyor belt and follow it under again to the right. Grab the key once more, then jump on the yellow block and toss the key straight up, onto the conveyor belt above.
Walk (don't run!) off the yellow block to land on the belt switch (quickly, though, before the key falls off the top belt). Ride the bottom conveyor belt under to the left, reverse the belts again, and hit the red switch (turn around at that point; there's no need to jump on the spring). The exit should then be wide open to you (just make sure you get to your key in time).
A well-executed run should earn you about 96 seconds.
The second part is actually pretty simple, too. It just has to be pretty precisely-executed as well (but that goes for most of this game, hm?)
There are two things to keep in mind about bob-ombs:
- They'll always jump out the opposite side of Mario. If you're trying to hurry a bob-omb somewhere, wait on the appropriate side (but not too close, or no bob-ombs'll appear).
- Wait until the BOMB blast is COMPLETELY gone. It's no fun dying on the second half of a level, and you're not in THAT much of a rush here.
That said:
Run immediately into that little hole to the left. The bob-omb will break the blocks. Use a second one to break the next blocks (of course), then grab a third one (wait on the cannon's right side), bounce up off the spring and toss it right in the nook of the blocks. While you're waiting for this bob-omb to go off, go ahead and climb up to the top, and run over and grab the present (you can run off the right side to get back, just don't fall ALL the way down at once).
At this point, a fourth bob-omb should just be poking out of the cannon. Go ahead and grab it, bounce up, and toss it down to destroy the blocks on the left (now, if you're feeling REALLY daring here, you can actually do a turnaround-jump off that bob-omb's head and grab the last present. But, again, you aren't in THAT much of a hurry here). Once you hit the blue switch, you can go back up and turnaround-jump to get the last present.
After that, it's just using the top bob-omb cannon to blow up the bricks between you and the Mini-Mario. Simple enough, no?
EDIT : Man, 6-mm is an annoying level to best... what's the target score on that one?
