Well, I wouldn't consider 5 1vs1 battles a boss battle - they are nice as a pre-encounter for a boss battle thought
Basicly, you can use any tank-like enemy as a boss fight or enemy that are considerably strong than any single member of the party, if you use anything else the party will have a easy time if the dice are on their side.
The tank option is relativly safe as he can be attack and defense wise close to the party so it will take them longer to take him down but the chance of one-hitting either side is rather low. The extremly strong opponent on the other hand should only be used if you have a good balanced party with chars specialized in different fields (tank, healer and damage dealer) that know what their jobs are and can stick to them.
To give tanks a little variation you can have high DR creatures, low-mid DR creatures with extra shields, high regeneration opponents with a decent armor and defense, or the good old high defense high at tank.
To spice things up for "strong guy" fights you can use high stats opponents with a weakness or give let them have closer stats to the players but have special defenses with the one or other hole to be found and exploited by the characters. Its also fun to first let the players a few fights against the boss without them knowing how to defeat him but letting them always a way out until a final showdown were either they already found his weakness due to roleplaying or have a last chance to find it during the fight (or get something that gives them what they lacked before). Or a multi part fight like you would have when fighting with omega.
Thats atleast how I handle those things.
Thought, there are times when I use actuall weaker enemies for a boss fihgt but thanks to having supperior equipment (artifacts intended as loot for the players) and powers that profit greatly from this, so that in the end the boss is stronger than the players.