DISCLAIMER UNO: this only applies to regular siege weeks, so during rune week, kula week etc. things are a little different.
DISCLAIMER DOS: if you are first to track a BB, all rules go out the window you just spam the shit out of it until you get a hit.
DECK: I never bothered to drop gilds so the only reliable way for me to farm is the 13,000,000 bosses with a simple Draador/Dendi setup. I am also too lazy to switch between decks so I use the same farm team to attack BB's. This means half my team dies before the first attack on a BB but that's fine. The only thing I change is I swap in the right 4M hero for the week and sometimes I even remember to gild him.
HORNS: it's a minor thing but I never use auto refill, it stops the refill timer and I figure in the long run that adds up. I manually keep 2 of the 3 horn spots filled in between fights.
TRACKING SPOTS: if you rely on bosses bing called in chat, never fill more than 3 of the 4 spots so you can react to what's being called. Other than that, you have a choice if you want to track new bosses even if a BB is already listed. It will never give an additional BB, but I find it gives me enough 13M bosses to make it worth it. Also, usually the rate with which bosses die is high during BB hour so you won't be bothered too much by having all 4 spots occupied.
[14:20]
SPEAKING OF WHICH: BB hour is the best hour for farming since you see so many bosses. So I always go for any 13M that still has half its health. Below that, at best you break even. Exception to this is when his health is very low and you need to clear a spot.
500M BOSSES: not worth the time or the horns. Either way they are usually gone by the time you click them. I also never call these, it just fills up chat and the chance of a clan mate being able to hit it are virtually zero. 99% of the time they are dead before the call appears in chat.
2B & 5B BOSSES: this is where the points are at. First thing to do is call to arms. The only exception to this is if it's already extremely low on health and you would risk not getting a hit in if you wouldn't attack it right away. Keep in mind some clan members can't track BB's themselves, they rely on your calls to score.
AND NOW WE WAIT: the first thing to do is nothing. You want a reasonable amount of damage to be done so the boss minions are all dead. Consider this when you create your deck, there is no use for multi-hitters when using this method. To be clear by waiting I mean going in and out of the siege list repeatedly to force the health score to be updated.
WE WAIT SOME MORE: usually you see the health slowly being chipped away at and then a sudden big drop. At that point you know some heavy hitter has joined the fight. This also means it will likely die real soon so now is the time to act. Look at the number of players that got a hit in (on the actual battle screen, you don't want to consider the number of players that have tracked it but have done zero damage) and decide if it's low enough to be worth your while. For the 5B bosses it almost always is, for the 2B bosses if it's approaching 100 players I save my horns.(edited)
[14:21]
BUT MARCH BY WAITING THAT LONG I'M OFTEN TOO LATE: this is correct. But overall it still gives me the best points for my time and horns spent.
AFTER THE ATTACK: quickly close the battle screen and scroll down the list to see if you got a hit. I have left my avatar blank on purpose so it's easy to spot. If you didn't get a hit and there's still time, try again. This is where always having two horns in place comes in handy.
BUT WAIT, THERE'S MORE: after a while you start recognizing the names of the big hitters. If a bunch of them are on at the same time, you don't want to wait for their first hit to go in because you're almost always too late. Wait for the minions to clear and then look up the number of players that got a hit, assuming it will die any second. Even if you don't recognize any names, after a few minutes you get a feel for the siege hour and can tell if that strategy is needed or not.
IF THERE'S LAG: in the ultra rare event of the servers being slow, after hitting the first attack button to join the fight, start clicking where you know the second attack button will appear.
AND FINALLY: I've never used power attacks other than for clan bosses. Using the above method gives me fine results without it.