So if you have played Gun Bros yet, you will notice that all the achievements required is grinding, grinding, and more grinding, its not so much hard as it is time consuming.
The best advice i can give is to have 10 gun bros so you have all bro buffs unlocked and to try to complete the bro ops as they add warbucks and other than signing in continuously and paying with real money, thats the only way to obtain them.
As for the "service x amount" and "service x amount with x weapon" achievements, the quickest way i have found is to just play through the horde rounds on the third planet as the zombies are easy to kill and the rounds are short.
As for buying stuff in the shop, I have not found a really fast way to earn money besides playing as many waves in one session as possible, because of the xplodium multiplier, also getting perfect waves adds a lot more explodium. Also you can buy 8 guns for less than 12000 i believe, and for 60 powerups you can just buy the band aids which would only cost you 9000. I have not unlocked the 20 armor pieces yet but im sure it would cost anymore than 15000.
As for leveling up fast, im not sure of any place or method that works better than just playing waves 40 through 50 repeatedly as those have the larger enemies which give 12 to 24 xp a piece.
So can anyone share their findings on how to speed up the process of grinding?