The attributes which one needs to work on are defence, resistance, unkillable and I am sure there are other parameters, it is very tricky

all heroes ofcourse have to be maxed out, using books, artifacts at level 16, and using materials that you get from fraction wars and all accesories.

I think it's normal if all of your champions are level 40 max, that any of the FW bosses are very hard still. If they were all at 60, the ones at stage 7 would be pretty easy all of a sudden. It's possible that you don't have the right gear (you mention high HP, but what does your defense and speed look like?), but at those levels, it may be very difficult even with good gear and if you're doing everything right. 

Ayumilove.net just updated their CB page.  Its full of good information about gear, skills and which heros to use.    

@L9753  is right about using 6-60 champs.  

Wait, When you say first boss, Do you mean the Stage 7 Boss or Stage 21 Boss?

I don't remember having issues vs. Stage 7 Boss.

The Stage 21 Boss is a nightmare.

The problem is the Stage 21 Boss Ignores Defense.

A few of my heroes have 40k HP + 4k DEF.

They get 1 shot by the Stage 21 Boss.

Yeah, its extremely tough

I still haven't been able to 3 star the Stage 21 Boss.