As greewing said, if you expect to succeed at level 42, but the stronger boss at level 43 will be problematic (because of available friends, energy etc.) then the summoning stone gives you a level 1 boss to fight for 11 gems to give you the 2 needed materials to be able to craft the boss+.
edit: but 6 levels in 2 days should be fine. Even when you'd need 3 friends per level that should be achievable and even using 13 energy per level that should also fit...
Good friends is very, very important. 1 friend should be doing much more damage than two followers at the higher boss levels. First time getting to level 43 seems to mostly be in the player level 90-100 range.