A tip of mine would be to focus on singular areas if you can. It's not so viable to do towards that start as you don't really have the stock of maps to do so, once you do though the 4 different conquerors can spawn one after another in the same region as you clear them out. Then you can stick the four watchstones you've obtained into another region and repeat. It feels better to me doing it this way rather than having a few different regions needing only one or two stones, then running the risk of a conqueror whose stone you already have spawning and eating more of your maps to clear.
Sirus is one of the endgame bosses, once you get to about 16 watchstones and capture the 4 conquerors his event will pop up. He then returns every time you capture all four of the conquerors. He seems pretty daunting on the first attempt or two, expect a few deaths going in blind. Once you understand how the fight works and how to deal with his attacks then he's honestly a cake walk.
On a side note, make sure you're always targeting the map completion bonuses when clearing the Atlas if you haven't already. You only need to do it once for each map, but it makes a big difference as you accumulate them as it gives a chance for maps to drop a tier higher, and eventually two tiers higher. That's making sure you complete each map with the following:
Tier 1-5 - Needs to be at least magic (Blue)
Tier 6-10 - Needs to be at least rare (Yellow)
Tier 11-16 - Needs to be at least rare & corrupted (using a vaal orb on a rare map)