Wanna batch-buy upgrades? Tap an upgrade three or four times and then next to it, a +10 and +100 button will appear, allowing you to mass upgrade easily. This is good for upgrading those weak warriors that you got at the very beginning who are cheap as hell. Since a ton of the earned diamonds require you to upgrade a certain amount of times (for achievement purposes), do this to make it easy.
Using the +10 and +100 buttons allow you to skip skills, too, if you don’t want to unlock skills before you upgrade as many levels as possible (for some reason). You can always go back later and unlock the skills, and if you have multiple skill upgrades built up you can unlock all of them at the same time.
Another really easy way to get all of the upgrade-related achievement unlocks? Prestige. Every single achievement carries forward after you prestige, so for achievements that require thousands and thousands of upgrades, hundreds of bosses killed and other massive numbers of stuff for you to do. It’s much more difficult unless you hack the game like some of the top players on the Game Center lists do.
Hacking the game requires nothing but a good hex editor. The easiest way to do it, if you go that route, is to download a hex editor and download the app to your PC (if you use an iPhone, no need if you play on Android platforms) and give yourself insane amounts of gold. Then speed-upgrade to the point that your DPS beats everyody immediately, and watch yourself shoot through levels.
Every time that you play through the game after hitting Prestige, the game will go by quicker, because of the artifacts that you have (most likely) purchased using the relics that you gained the last time that you played or prestiged.
When you are redoing a boss battle, you have to hit the fight boss button to do so, but try never to hit it when you have another enemy almost killed, ESPECIALLY when that other enemy happens to be Chesterson (the gigantic treasure chest). Chesterson happens to be the most lucrative enemy to kill, other than bosses, so NEVER let one slip by.