I believe that this specific format is the one where you can be the most consistent. The idea is that if you receive good boards then it really doesn’t matter - beyond a reasonable limit - what heroes you brought to the fight, so you only have to prepare for bad boards.
With a mono team you can decimate your opponent in a couple of moves if you get good tiles, however that same board would most likely give you the win even if only brought 3 from your color, it would just take longer. That gives you the opportunity of bringing off-color heroes who heal you in case of a bad board where you don’t get tiles for your 3-stack.
Obviously in Bloody Battle this does not work and in Rush Attack you won’t have the time to recover, therefore Buff Booster is the only format where this strategy is consistent.
5* tournies have devastating specials that also make it hard to recover and even 3*-s have very strong snipers (Namahage, Gato, Bane, etc) who can 1-shot your heroes after a couple of slash attacks. However 4* snipers are generally not that devastating, they either have high base attack but low % on their specials (Grimm, Scarlett) or the other way around (Caedmon, Sonya). Therefore it is rare for your heroes to die early (unless of course you have no costumes, no emblems and go up against +100 teams).
Finally, yellows don’t have much in the healing/cleansing/debuffing department, making the color restriction almost meaningless.
Combining these elements I think that a 3 healer + 2 sniper setup is the most effective in this format and I am 20-0 on attack using this principle. The team I used in my last 15 attacks is the following: costumed Rigard+19, costumed Boldtusk+18, Kiril+18, Sonya, costumed Sonya
This gives me tile damage, heal in 3 colors and cleanse/debuff that can be adjusted based on the opponent. On the first day of the tournament I have experimented with Grimm+19, Peters+19 and costumed Melendor (even tried out 2-2-1 teams) but in the end survivability won over emblems and Sonya is just amazing at that.
I had many bad boards along the way but heals were always just enough to keep me alive until the next one came online and focusing down enemy heroes one by one always allowed me to come out on top. Slow and steady wins the game.
Regarding defense I honestly don’t know what is the best, I took down Cyprian, Rigard and Tiburtus tanks alike who all tried to do different things. My own defense went C, B, A and now it is back to B, I have my 3 emblemed healers as tanks/flanks, Peters and Grimm as wings. I’m pretty sure it’s not ideal but I don’t think that there actually could be an ideal setup.
I’m currently just outside the leaderboard, with a straight A defense I would be at #3 right now but that is an element we can’t control.