Although this question has been answered, I'll offer a strategy that I have used with much success once my bro gained more powerful weapons and armor.
The key is to ensure that you've logged in through Game Center, and that you start a marathon session (maybe 1 or 1.5 hours total) on wave 49 or 50 of a revolution.
By starting on a later wave, you're guaranteed to kill a ton of enemies, and (if your character is powerful enough) guaranteed to survive through to the next revolution.
Once you've completed a revolution, the beauty is that the next waves are really simple -- at least 10 - 15 of them are. Make sure you don't get hit, because you want to accumulate the 10% xplodium bonus. This bonus is compounding on your current xplodium total, so you've just taken the xplodium earned (a decent amount) from waves 49 and 50 of the previous revolution, and gotten 10 - 15 consecutive 10% bonuses by breezing through the first waves of the next revolution.
Ideally, you'll have started off with several health nukes so you can replenish your health as necessary. I purchased enough to go through waves 49 and 50 of one revolution, complete the entire next revolution, and then the next 30 waves or so of the following revolution. At that time, there was a 400% bonus enabled in the game, so I got even more at the end. Then I stuck that into one of the high-multiplier refineries to get 2-4x the amount! That's why it's important to play through Game Center.
Just to give you some numbers, when I used the 4x multiplier, I had 17k xplodium (for a total of almost 70k). And that was unfortunate, because on my next marathon game I ended up with 80k xplodium, and only had the 2x refinery available.
Hope this helps. It has really helped me purchase some cool stuff.
Best Answer
Your default bro will not upgrade to better weapons, armors or higher number of hitpoints. However, you can connect via GameCenter to other players and use their bros, with upgraded weapons and armors. The game gets pretty hard without using other players' bros.
You can find other Gun Bros players here, as an example.