Starting out early to mid game
This guide shows how to start out with ascension and not make mistakes and also how to be efficient with fodder. To start with ascension is crucial for this game as it increases the cap of the hero, so if a player makes bad decisions then he might be roadblocked by the hero cap. So to start with let's talk about early game to mid game (1-160 hero levels) ascensions. Here you most likely won't have many ascended tier heroes and it's fine. In this period of the game legendary heroes can still carry you, so you should be aiming on getting Silvina/Vedan, Saveas, Hogan, and Arden to Legendary+. These are one of each factions legendary+ tier heroes as they are rare they can be easily ascended to Legendary+.
Reasoning behind first 4 hero choices
Saveas because he is best dps solo carry early to mid game, hogan because he is good tank and Arden because he is best cc (crowd control) support. Also a good carries can be Silvina and Vedan but they are not as good as Saveas so primarily we still get one of them to Legendary+ as they can be used on Shemira later on.
Starting to plan out Ascended tier for late game
Now after getting one of each Faction Heroes to Legendary+. You need to look at how many ascended tier hero copies you have. Let's say you have 2 Elite+ copies of on ascended hero, this means that with enough fodder you can take them to Mythic+. This way you make sure that you at least have a resonating crystal at level 200 with 5 mythic+ heroes.
Strongly suggested heroes
There are 2 stores in the game that offer specific ascended tier heroes. And a new biweekly event Voyage of wonders has a chest with 4 ascended tier heroes choices. This comes in handy as you are able to plan your ascensions. At once you can be working towards 3 heroes that you want to get to ascended.
Top picks from Labyrinth store:
First would be Shemira as she can carry you really hard after 161 level. Second would be Nemora as she is superb support and a healer.
Top picks from Challenger store:
Best choice is between Grezhul and Fawkes as they are both really strong in certain situations. Unless you already have close to 8 copies of any other heroes from the store it is advised to buy those to be able to take them to ascended.
Top picks from hero choice chest:
First is hands down Lyca as she is a superb support with good damage and can be used as dodge tank in late game. For the rest it is the same as challenger you want to look at how many dupes you have of those heroes and decide.
To always plan ahead and aim to have a solid plan for first 5 ascended heroes and not to make a lot of Legendary+ heroes as that consumes 2 elite+ fodder which could be used to take heroes to ascended once you know that you can get guaranteed copies of them.