Man I love this game! I've figured out a lot of useful things, so here's a list of important stuff~
1. Stealth is, in fact, much better than open conflict- ESPECIALLY on "Survive" levels. Do not, at any time, use your gun unless you see the crazy pig(s).
2. Fear the crazy pigs. They got a gun AND you need two shots to take them out. I believe they're always in a building with a massive stash of resources just behind the corner, so if you REALLY need ammo, healthkits, and food, go for it. But make sure you have about 4 bullets before you go in there, god knows how many pigs there'll be inside.
3. Easiest way to take out crazy pigs is to lure them out by standing there (like a doink) and making them go outside. Then, find a bush and get down before he comes out to look for you and shoot them- it's a lot easier than running around like a crazy pig.
4. insert angry birds sequel joke here