Final Fantasy d6 (Complete System)

[Vicedets]

I'm running a game and my party hit level 4. They got to a boss fight, and between Mad Rush, Shiva, Backstab property, and a whole list of other abilities, dealt around 550 damage within two turns, and killed it on the third.

I have a group of six players, and there was only boss, but so far in my game, one boss has been able to take them, so the player/monster ratio was fine. I want the party to win, but I want there to be a challenge. Is there any other way to make the boss fights harder other than adding a second boss? I looked at it like I would an actual FF game, where it'd only be three or four characters on the field at once vs a boss, so there'd be three or four actions for every boss action compared to where I'm at now, which is six (12 with my warrior's Mad Rush) vs 1.

Is there a good ratio for monsters vs players and bosses vs players? Or am I just not building my bosses strong enough?