Currently the whole achievements tab is a one big mess. Its hard to navigate through it and its hard to check particular achievements fast. And there is no button to filter it.

How about to change the look of some achievements? For example, there is a lot of them requiring you to beat particular wave once. Why not making a single one 'Beat wave X' with many levels? For example, the first level asks you to beat wave 5 to get bonus, after that achievement just updates itself to level 2 and gives you new mission - clear wave 10, etc... Its a quite simple thing and will cut the size of achievements tab, making it a lot easier to navigate.

The same goes to those 'get X coins' and 'kill x enemies'. This also will make it quite easy to add new ones over the old ones just by adding to the top of it, instead of making a whole fresh ones.

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Another suggestion — have you considered adding some active element to the game? For example, in Cookie Clicker there is a special cookie appearing from time to time (per each 5 minutes or so) which grants extra benefits for those who is active (+% extra gold found for X seconds or just gives bag of coins instantly). There is also one giving you a lot of coins for manual clicking, but for very short time (i thinks, like, 9 seconds or so), this will make using some skills and bonuses a very nice addition, creating synergy.

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The last one. Wait for it xD... How about making rewards system for getting particular ammount of idols? For example, when person reaches 15 idols, he/she gets + 2 small chests. For 20 idols you get random piece of normal gear, etc. For later stages you can even lock getting new heroes untill you get some ammount of idols (lets say, 100). Alternativly, the idols can be used to improve starting time of any hero by 1 hour. This mean, each 5 idols given to a hero will boost his attack by 100% as if he played those 5 hours. Works upon each reset. Note: this way used idols will not be lost from your inventory - you can just give them some active use.

Any chance of making the achievements that require x number of monsters and bosses work while griding (or whatever its called while we aren't playing the game) I know the money achievement is like that now so can the monster and boss achievement be upgraded to do the same, its really hard to just leave the game on a level to do things like that.