Before attacking in the wild, check rankings. It will give you a basic understanding of what you're in for (not all the time).
Time your upgrades so you'll have what you need when the upgrade is complete.
Never keep a large amount of coin/cred on you. There are always players lurking to take it from you, and you need it more than them.
Work on getting a full team before trying to boost all your heroes to Saints.
Save your Spirits for said needed heroes.
Have no shame in farming others, it is a war game and it is competitive. If you need the coins/creds, you have to work your butt off to get them.
Never stop trying to cross in Pirates, chances do come and you will get across.
Invest in Guild Techs. They help more than you know.

I could make a long list of the things to keep you in top spots, but chances are, everyone will tell you these things if you just ask them. People pay to get an advantage over you, so you have to work hard to keep up. I keep in top 10 in Arena and am only VIP1, and do not buy anything at all. It isn't impossible to do, just tough - And you know what? If you can keep up with high spenders and you don't have to spend, what does that say about you? You win some and you lose some, and there will always be people better than you. Is just the way the game works. I hope you figure out how to keep up and have fun with the game!