A very important point that you absolutely must understand in order to play intelligently: boss drops are not random. At all. At least with the possible exception that if a boss is going to drop a shard, the type of shard it drops might be random.

Each level has 15 opponents: 14 normal and a boss. In most cases, the boss is just a normal boss. There is a small chance that it could be one of three types of special bosses, however. A "ShardKeeper" boss drops a shard, a "Super Boss" drops a heart of eradine, and a "Treasure Whore" drops 20x the normal copper.

All three of the special types of bosses are labeled on their name. A ShardKeeper boss looks like a normal boss other than the name. A Super Boss is bigger, and a Treasure Whore uses a particular graphic rather than just being a random boss.

All three of the special types of bosses are also much stronger than normal. A ShardKeeper boss has perhaps 2x the normal HP. A Super Boss has perhaps 3x or 4x the normal HP, but also gives you double the time to beat it. A Treasure Whore has something like 4x the normal HP and without giving you extra time. I'm not sure of the exact numbers, but they're all much stronger than normal bosses.

This means that if you're barely strong enough to beat a normal boss, and a special boss shows up instead, you're going to fail unless you can burn some skills with long cooldowns to beat the boss. You won't use such skills unless you're sitting there watching it, and idle games aren't meant for you to have to watch them constantly. That means that it's critical to be much stronger than necessary to beat the bosses you're facing, as otherwise, you basically don't get loot. You'll get some copper and that's it.

If you fail against a Treasure Whore, it sends you through to the next level, anyway. If you fail against a ShardKeeper boss, it does not. I haven't watched a failure against a Super Boss, but my guess would be that that also stops you. Treasure Whores are massively stronger than the other types of special bosses, and also give massively less useful loot.

One consequence of this is that, even if you could push through and barely make it to another threshold before rebirthing, you really shouldn't unless the next threshold gets you an extra heart or amrita. You're not going to get any more loot on the way, so you might as well just rebirth. Another consequence is that the coin boosters are far more valuable than they appear at first, as it means you'll level out ahead of the content and be massively stronger than the content so that you can easily clear whatever special bosses show up.