The hatching strategy
The best egg hatching strategy depends on what stage you are in the development of your collection. I’ve categorised these below and you can decide which is most relevant to you and when to progress from one strategy onto the next. I’ve written them following on from the previous stage so be sure to read through them all.
In a nutshell:
You hatch on eggs where you don’t own the featured monsters and in the later stages you progressively shift focus onto mythics by worrying less about the quality / how many of the featured legendaries you have already, once you’ve got a good collection.
Early stage
This is where you have a fresh account.
Hatch when you own none of the featured legendaries and there’s at least one top legendary in there (some kind of S tier - tier list). As soon as you hatch a featured legendary then stop hatching. Focus less on the mythic but if there are many eggs which meet the condition then hatch on ones where the mythic is a monster you’re interested in or where there are 2+ high tier legendaries being featured.
Middle stage
This is where you’ve got enough good monsters you can complete PvE events and do fine in PvP. This shifts focus onto building collection size and awakening mythics. You can choose to skip this stage.
Continue to focus hatching on eggs where you own none of the featured legendaries but now care less about there being a high tier monster. Instead, do it on ones where the mythic is high tier or a mythic you particularly want. Again, as soon as you hatch a featured legendary then stop hatching. What this will do is slowly work towards the good mythics while still building your legendary collection.
Later stage
This is where you own many legendaries so too few eggs come under middle stage conditions.
Create a list of the mythics you want to awaken that is maybe 5-10 mythics long. Hatch only on eggs featuring these mythics and no more than one legendary you own. Stop hatching when you get one of the legendaries you’re missing (or if you awaken the mythic, obviously). When you awaken mythics and the list is getting smaller, consider adding more onto it.
Note: This is going under the assumption there are three legendaries featured in the egg and it’s not a festival. If there are 4+ legendaries featured then consider continuing when you have 2/x until you have 3/x (x being the number featured). If it’s a festival then assess the egg based on the displayed drop rates of the monsters you don’t own relative to a normal egg you’d be happy to hatch on (get a calculator if you need it - add up the drop rates).
Spender / Ultra late stage
This is where you’ve got practically all legendaries you care about or are happy to spend for mythics.
Create a list of the mythics you want to awaken that is maybe 5-10 mythics long. Hatch only when they are featured and when there is at least one featured legendaries you don’t own. Once you have all the featured legendaries then stop hatching. Bear in mind this is only a recommendation for efficiency. You can obviously just go all out until you awaken the mythic if you wish. If you’re a spender who plans to do this I recommend making your list slightly smaller; more like 3-5 mythics at any one time. This will probably help you spread out your spending and avoid hatching ones you regret spending on. It will give you time to really get the most out of the mythics you awaken and learn to use them well.