Alright, first time playing it right, in "free play", i really need a guide. How can i "beat" a portal? i completed the exploration of the first one and some "azure ocean", but nothing special happened. Then, how do i get "LightningStorm", and finally. how hard are the final bosses?? i found some videos about the overseer but nothing more.
So, the final bosses, Evo (Evolution), Crea (Creation) or Druid are rather hard if playing solo, rather easy if playing with a lv 20 team that you know how to use.
World bosses are seriously hard, due to special terrain mechanics.
Hidden bosses are even harder either due to special mechanics (Immune to damage until X happens) or bloated stats and debuffs to your team.
Free play shouldn't be your first real playthrough. A short 15 or normal 30 day is probably balanced right. On the last day, a special portal opens with the final boss. If you've done quests and other things correctly, you could access other endings and bosses instead, but keep that for future runs. I might be wrong, but to even have a chance to get to those, you need to finish the game with each of the main 3 characters, since there's information that each of them gets that put together, can lead you to those extra endings.
As for Lightning Storm... someone else will need to tell you. I generally quite like fusing and breeding characters more for the achievement than play. I prefer having a team of Uniques/NPCs