This video covers the third level ("Act 3: The Mission") in the METAL GEAR SOLID® Premium Level Kit for LittleBIGPlanet. Since it is possible and not too difficult to do, I Ace the level and collect all of its Goodies in one go.
If you trigger one of the Searchlights, an alarm will be triggered, nearby doors will close, and you will be fired upon until you either somehow escape or are killed. Additionally, triggering Searchlights can deny you access to some of the Prize Bubbles as a door will shut over the entrance to the alcove in which they are hidden.
The tank mini-boss can be a little difficult (and probably is the biggest obstacle in the way of Acing this level), but it's not too bad if you use the alcoves to hide. Although you are not explicitly told this, you can use the Paintinator's paintballs to detonate Impact Explosives (which include the ones the tank mini-boss launches). Also, make sure you are far away from the tank mini-boss when you defeat it because the resulting explosion can easily kill you if you are too close.
Since it is possible to get all three Medals in one go, you may be wondering what the Sticker Switch at the beginning of the level is for. Completing this level gives you the Solid Eye Sticker that you can use at the beginning (upon a revisit, obviously) to activate a Race. Unlike in "Act 2: VR Training", no new Prize Bubbles appear once the Race is activated. Instead, it serves only to offer you extra points for finishing quickly. If you'd like, you can go for the Race option whilst going for the Thrifty Painter Trophy to earn some points while going for an extra optional challenge.
Speaking of the Thrifty Painter Trophy, I do not think I will do a video on that. There are some videos by others on that out there already and I also faced more difficulty than I anticipated and remembered compared to when I originally got that Trophy. The gist of that optional challenge is to finish the level using 125 paintballs or less. It's not too difficult (though you should anticipate dying once or more due to the gauntlet of Plasma balls you'll have to pass), but precision and conservation is vital. The mini-boss at the end of the level takes a fair number of paintballs to defeat (I'd estimate somewhere in the ballpark of 100 or perhaps more), so you should focus on conserving most of your limited shots for that. If you're very precise and waste very little or none of your shots, you can probably safely spend your paintballs on ONE 10-shot enemy (the first one is a good choice as it can be tricky to jump over) and all of the one-shot dropping platforms. You should still have enough to defeat the entire mini-boss if you continue to be highly precise and accurate. You can also easily jump over or bypass the vast majority of the turreted enemies in this level, so take that opportunity when it arises to save even more ammunition for the final fight. If you succeed in using 125 paintballs or less, you'll earn the Trophy as soon as you step onto the Scoreboard. Good luck!