Yeah, I've never gotten the fire extinguisher trick down.
As for easy, non-silent assassin ways: there's always standing behind a wall, popping off a shot with your most silenced pistol, and then running like hell when you run out of all other ideas. Or (and I literally had no idea this worked until today) conceal an explosive in a trash can and set it off at the right time?
As for strategy, here are ten random tips (although any of this could change in future patches):
1. Any car, motorcycle, or lawnmower is an instant accident kill by explosion if you can shoot the fuel tank (you need to learn the right spot to shoot for each of these), then shoot the pool of leaked fuel that comes out. Or just set off a strong explosive next to it to blow it up without making a fuel leak.
2. Light melee weapons like soda cans, pool balls, and toy tanks are always better than heavy ones like wrenches and crowbars because they are faster for both striking and throwing.
3. Immediately after noticing a distraction there is a small window when an NPC becomes oblivious to almost everything. You can get away with a lot during that time.
4. If more than one person hears a distraction, it will be random which one goes to check it out (and the randomization is heavily biased towards non-targets). Either put a distraction in a place where only one person can hear it, or use multiple distractions where there is no other way to do it.
5. Distractions work across any vertical height, as long as the sound can reach. You can get a person to go down multiple floors on their own if you get them to hear something that lands on another floor.
6. The Silverballer gives you perfect accuracy across its full range if you squeeze the right trigger halfway. The Krugermeier is super-silenced so that even guards within earshot will not identify the noise as a gunshot. Both have their uses in different situations.
7. For guards, a weapon on the ground is a better distraction than a coin. It lets you control where they go and force them to leave their post and walk to the nearest weapon storage box. Explosives and audio distractions have an advantage here in that you can throw them.
8. In most levels there are two tiers of guards. The "unprofessional" guards don't communicate by radio, and the "professional" ones do - but only within arbitrarily predefined "teams", I'll call it. This is kind of important for elusive targets, since one trick they use is defining all the bodyguards across the whole map as part of the same "team", so you can instantly become compromised across the whole map (which never happens in story missions).
9. If you're sabotaging or disabling anything, shooting it from a distance usually works. Most commonly missed: shooting electrical outlets instead of using a screwdriver.
10. Almost anything can be a distraction. Shooting an unsilenced gun, setting off a car alarm, blowing up anything, walking into view of a guard for just long enough to make him investigate... Often it's harder to knock out guards than to just distract them.