(play Level 180 without heroes so that the Boss does not spawn mini-bosses. Caldera does not die because he is supposed to be immune to physical damage.
During Level 140 after Caldera comes onto screen, use heroes that stay alive/tank him but don't die will allow you to play this level almost endless mode. Avoid damaging the Boss will keep the level going longer World 5 Towers: Name During Level 90 or 110 after Boss comes onto screen, by placing your towers specifically, you can ignore everything else and use some empty slots to place, delete and place towers over and over again until the Boss dies. During Level 50 or 60 after Boss comes onto screen, if you do not engage with him, you can play this level forever which will help you kill enemies, use hero abilities but also allows you to place, delete and place towers over and over again in empty spots. To slowly farm - at last wave of your easier daily RS maps (once you can ensure afk win), just sell one tower and keep bulding and selling your least used tower of that world on that tower spot, while you slowly win. To grind fast - use Endless Mode (each endless mode starts with 2000 coins) to place, sell and place towers again once you have all the towers set up. Each tower has three achievements regarding how many times they are placed: Some towers do have map levels that will allow you to place, sell and place again easier than others. Each world has 4 tower types which have achievements.