I would argue the game is divided into four phases. The boundaries of each are

Early - Brutal 12-3 or Brutal 12-6 on auto farm in sub-minute times

Mid - Brutal CB on at least 3-key

Late - NM cb on at least 3-key

End - UNM cb on at least 3-key

Early: Farmer to 6* for 12.3 Brutal

Mid: Dragon 20, other dungeons 17+

Late: Arbiter missions done, 80%+ on FW, 70/80+ Hard Doom. If monthly fusions/fragments are easy you are probably here.

End: Completed Hard DT (maybe not all secret rooms), Lydia (or very close), Gold Tag Arena 

I would classify myself as very Late game, 110+ in DT each month. Only 4 or 5 factions not at stage 21, though 2 haven't beat 14 LOL. Sliding between S2 and S4 in Tag (if you think classic is tough, ...).

From another angle:

Early: I don't have a fkn clue what I am doing

Mid: I never have enough energy

Late: I don't have time to use my energy, keys, tokens, ...

End: My 3 Septimus got nerfed, my dungeon runs are longer than 30 seconds now, but when are level 30 dungeons with no rare gear coming out?

I  feel  like  the  game  has  expanded  to  6  different  phases  vs.  4  phases.

My  way  of  thinking would  be  the  following:

1.  Early  Game  -  Things  which  require  1  Strong  Hero  to  win  vs.  a  Team  of  Heroes.

Campaign -  Normal Mode,  Hard  Mode,  &  Brutal  Mode

Faction  Wars  -  Stage  1  to  3

Early  game  is  about  focusing  on  1  hero  and  making  that  hero  the  best  they  can  be  as  an  individual.

Example:

Starter  hero  like  Kael

A  lvl  50  Kael  would  be  able  to  Solo  Hard  Mode  of  Campaign.

A  lvl  50  Kael  would  be  able  to  Solo  Dark  Elf  Faction  War  Stage  1  to  3.

Keep  in  mind,  we  are  not  building teams  here.

We  are  just  using  up  Energy  +  Faction  Keys  we  get  from  Dailies.

2.  Pre-Mid  Game  -  The  Transition  from  a  Solo  Hero  mindset  to  Team  mindset  

Clan  Boss  -  Easy  Mode  to  Hard  Mode

This  is  the  first  step  in  learning  how  to  use  heroes with  different  individuals qualities and  trying  to  make  them  function  successfully  in  a  team  environment.

Clan  Boss  is  the  most  researched  & talked  about  thing  in  the  game.

Clan  Boss  teams  allow  players  to  think  in  terms  of  "Roles".

In  my  experience,  Players  which  skip  Clan  Boss  suffer  from  the  "Roles"  concept.

To  build  a  good  CB team,  You  need  heros  which  offer  something different to  the  team.

Hero  A  "Role"  in  team  is  to  do  Decrease  Attack

Hero  B  "Role"  in  team  is  to  do  Decrease  Defense

If  a  person  is  Good  in  Math,  but  Bad  in  English.

You  pair  them  up  with  someone who  is  Good  in  English ,  but  Bad  in  Math.

Why?  Because  they  cover  each  others  weaknesses and  that  is  a  successful "Team"

Clan  Boss  is  the  Foundation  &  the  Back  Bone  of  all  things  in  this  game.

3.  Post-Mid  Game  -  Dungeon  Teams  +  Faction  Wars  +  Arena

Potion  Keeps  -  To  Ascend  your  CB  Team

Minotaur  -  To  get  Masteries for  your  CB  team

Dragon  -  To  get  Better  Gear  vs.  Campaign Gear  for  your  CB  Team

Spider  -  To  get  Accessories for  your  CB  Team

Ice  Golem  &  Fire  Knight  -  To  finish  Missions

You  begin  to  see  how  the  previous steps  begin  to  stack  on  top  of  each  other.

Step  1  =  Teaches   you  to  spam  out  6  stars  faster

Step  2  =  Lets  you  learn  about  what  heros  to  6  star  in  a  CB  team  setting

Step  3  =  Build  up  the  CB  team  with Potions,  Masteries,  Better  Gear,  &  Accessories

Faction  Wars  plays  small  role  here  due  to  Forge.

The  Forge  offers  gears  which  are  Clan  Bossed  related/useable

-  Perception

-  Resilience 

I  wouldn't recommend building  Faction  War  teams,  but  I  do  think  a  player  might  begin  to  make  some  progress.

You  are using  lvl  60  heroes in  CB  -  You  toss  them  into  random  Faction  Crypts.

You  will  be  able  to  beat  some  Stages.

In  my  personal opinion,  You  don't really  need  a  Faction  War  team  till  2nd  Boss.

Stage  14  to  Stage  21  can  be  very  tough  and  you  might  need  a  strategy to  win.

Stages  1  to  Stage  13  are  fairly  winnable  with  out  a  team.

In  most  cases,  you  need  1  or  2  strong  heroes which  can  carry  your  team.

You  can  beat  alot  of  stages  with  out  trying  to  beat  the  stages.

Example:

Tayrel  is  amazing  in  Clan  Boss  for  Decrease  Attack.

You  build  Tayrel  for  CB  in  Lifesteal  gear  to  help  your  CB  team.

You  Tayrel  is  lvl  60  +  Full  Masteries  +  Full  Ascension  +  Fully  Booked  +  Max  Gear

Lets  not  beat  around  the  Bush  here.

Your  Tayrel  is  going  to  start  wrecking  Face!

You  originally built  him  for  CB.

You  spent  all  your  time  &  effort  making  the  team  better  &  better  &  better.

The  result  is  the  heroes will  begin  Flexing.

Tayrel  presence is  starting  to  be  felt  as  side  effect.

You  built  Tayrel  up  so  strong  that  he  is  imposing  his  will  in  other  area's  of  game.

Your  not  even  focused  in  other  area.

It  is  simply  happening.

This  also  transition to  Arena.

Tayrel  can  be  great  for  Arena  for  AOE  Def  Down.

The  benefit  of  the  champion  can  be  wide  spread  in  various  things.

  

4.  Pre-Late  Game  -  Acquiring Arbiter  +  Faction  Wars  +  Normal  DT

At  this  point ,  you  have  got  Arbiter.

-  Normal  to  Brutal  Campaign  is  done

-  Clan  Boss  is  done

-  Dungeon  Stage  20  is  done

-  Arena  Gold  4  is  done

What  you  need  to  work  on  now  is  getting  Lydia.

-  Faction  Wars  -  Full  3  stars

-  Normal  Doom  Tower  -  Floor  120  Completed

5.  Post-Late  Game  -  Acquiring  Lydia  +  NM  Campaign +  Dungeon  +  Hard  DT  +  Tag  Arena

At  this  point,  you  have  got  Lydia

-  Normal  to  Brutal  Campaign  is  done

-  Clan  Boss  is  done

-  Dungeon  Stage  20  is  done

-  Arena  Gold  4  is  done

-  Faction  Wars  is  done

-  Normal  DT  is  done 

What  you  need  to  work  on  is  getting  Ramantu.

-  Nightmare  Campaign -  Full  3  star

-  Dungeon  Stage  25  -  Auto

-  Hard  Doom Tower  -  Floor  120  Completed

-  Tag  Arena  -  Gold  1

6.  End  Game  -  Acquiring  Ramantu  +  Praying  for  New  Content  +  Arena

At  this  point,  you  beat  everything  in  the  game.

The  only  thing  you  have  left  to  do  is  pray  the  game  releases  something new.

The  only  joy  you  get  as  End  Game  Player  is  Arena.

As  of  right  now,  No  one  is  End  Game.

All  the  previous End  Game  players  are  considered Post-Late  Game.