This game is great as far as idle games go, but the inability to go back to previous stages is a huge overlook. You're forced to very slowly farm a boss, which slows the game to a crawl.
Since you can grind on the boss we weren't sure if the ability to play a previous level was needed. However due to all the requests for this so it's something we're looking at adding in a future update. EDIT: Update 0.6.2 now allows you to grind the wave prior to a boss.
to beat the first boss put a point in armor and triple cannon. you'll want to build 4 or 5 cannons by the time you hit the boss. Then just send as many raids as you can against him. You should be able to take one hit from him with the added armor
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Since you can grind on the boss we weren't sure if the ability to play a previous level was needed. However due to all the requests for this so it's something we're looking at adding in a future update. EDIT: Update 0.6.2 now allows you to grind the wave prior to a boss.