making 6* in late game is not that hard even for F2P. I'm a semi-F2P, have not do any purchase lately.
every month you can get:
5-7 4* rainbowmon (1 each week from guild war, 1 from toa normal, 1 from toa hard) - this makes making 6* easy
1 12 hours double EXP from daily login (and 1 more when you make a 6*)
plenty of free crystals (10 from daily quests, maybe 50-100 per week from fighting rivals, 100 each week from arena just to finish in fighter 3, 700 from TOA normal, 190 from TOA hard if you finish level50, daily login few 1-200 (forgot how much) ---- that's 10*30 + 70*4 + 100*4 + 890 +150(?)= 2000+ crystals per month! use them for energy refills
i just made my b10 necropolis team - takes 3mins+ with above 60% success on auto:
zibrolta lv30 (lead), bella, hwa, yen lv30, colleen (only 5*)
turn order: bella, hwa, zibrolta, colleen, yen. i did it this way because i didnt rerune my +180 speed bella & hwa (should be able to replace hwa with a slow hitter or a fast multi-hit shield breaker too)
That's about what I do, but roughly every month just can level one monster to 6 stars on average, of course I can finish 1-2 more 5 stars fodders sometimes when there's nice event going on. I do sometimes make 2 6 stars in a month. But then on average only 1.