OK Sam - but just for YOU would I give away my top secrets lol My highest score so far was around 45,000 don't remember exactly or the level it was. Here are my strategy tips:
Of course kill as many of those darn birds as you can on each level and clean after each level
Upgrade to the second weapon asap as long as you have about 50 total bullets in both weapons ( you can change between weapons if you run out - usually in lev two I start with the lesser weapon and use up those bullets until the crows come by )
Level one through 5 you don't need much more than 75 bullets so save the rest to upgrade your weapon to the third level & don't add protection yet
Get that gun as soon as you can and still have about 75 bullets level five -- generally-- I watch how many birds I'm getting and add about twenty - fifty more bullets per round after that while remaining at the third level gun. there are plateaus in there where-- you will only need say 150 bullets for a couple of levels then move up to 175 - for example but I don't have that kind of thing stored in my head lol but I never have more than 200 bullets before I buy the fourth level gun
So after cleaning and buying the bullets you need as you go and getting the third gun start adding all excess to defense ... when you have around 50 defense this is where it gets trickier -- but if you have enough money to clean, buy level four gun and at least 400 bullets for it and still keep about 50 bullets in the level three weapon then go for it at that point -- you may have more in defense by that time
Then each round clean buy increasing bullets up to around 700 - 800 over the next levels -- you have to really practice to control the shooting on this gun so as not to waste a lot of bullets when you shoot this is also a critical thing - and then keep adding defense until you have 100 percent
it takes a few rounds after that to have the money to buy the big boy but believe me you will need it
from that point on clean - keep the defense up and buy as many bullets as you can. the strategy then is to figure out each level what to use and when
always keep some bullets in level 3 4 and 5 weapons if you can -- there are some creatures best tagged by each one in higher levels
You will see about how many bullets you use per round as you progres through the game and next time you play only go for that many saving the rest of the money for defense or weapon upgrade
there easy see .. lol
ps the defense works in that it takes say like ten more times for you to get hit before it registers on the scale - it isn't passive it just isn't as apparent at lower levels