UPDATETIn the Newest update this solution did not work properly also the monster slots are glitched again if you managed to buy those beasts you did not get the achievement -.-
This is now Fixed first of all download the update.
if you have all 4 slots full of monsters you need to delete one of them and add them again the achievement pops then :)
To get this achievement you need a total amount of 1.200.000 (?) coins
100.000 for the second monster slot, 500.000 for the third monster slot and 600.000 for the fourth monster slot
the fastest way was found by TanukiV
Play the 3, 4 or 5th lvl (the lvl you are most comfortable) with an high lvl Adventurer with a high lvl Shield Aura, Stone Defense and MAX Treasure Magnet skill. Also use the Double Income Special (3000 Coins for 10min) (the Transmute is useless because the adventurer gets an auto transmute for all coins in Fever mode.)
And run through the lvl get in fever mode as fast as the game let you and you can rack up easily 10k+ coins in each run. Each run cost round 2min 30sek. Boss not needed but if you leave game your progress get lost. Die or finish the lvl to collect the coins
if you have no adventurer a easy way i found to rack up coins is replay lvl 1, you Can play any lvl you like i checked this twice it makes no difference, over and over again with two active specials the "Double Income" (3000 coins) and "Transmute" (6000 coins) special.
You can use the specials for 10 mins. this is 4 and a half run in lvl 1. each run are round 2mins 15 sec with the Gunslinger.
With both of the specials active you collect 17000 coins if i subtract the 9000 coins the specials cost its 8000 coins in 10 mins.
With only the Income specials its only 5000 coins in 10 min.
With no special 2500 in 10 min.
You need a double jump to jump on the "air bridges" the walkways in the sky you can see from the bottom to collect the 25 up to 30 gold coins on top on theses ways. if you do not collect them you get round 1400 coins less then in a 10 min run.
after round 120-800 rounds (depending on monster) you have collected the total amount of 1.200.000 needed if you spend them for nothing else ;) and you get the finish 100 rounds achievement also.
the coins only count if you die in the round or kill the boss keep that in mind!