well, it's much time since i watched DBZ for the last time. but as i remeber sohuld be this way:
Freeza: it would fly around in circles.
Every 20 seconds he stops and build up energy ( that KI "flames") during 3-5 seconds it's the main opportunit to attack him without risk.
always that the player get near he will do an energy eruption like attack that pushes the players back slowing them for a while and doing damage.
*normal attack: he flys without stop moving "pewpewing" the player with his pink laser shots from the fingers. ( these are fast since go straight foward)
*usually he stops and send an homing energy disc ( medium speed, but don't last forever, an skilled player can avoid them until it desapear)
*also at random, he teleports to near the player and gives an sequence of attacks, then teleport back to where he was.
*Every 10% HP lost he stops flying and concentrates an energy ball above it's head for 3 seconds, then unleash it against the players, it ill be homing, HUGE and slow but don't dissapear, the players ill must attack it ( will be targetable, i guess an suicide unit with special effects around could do the trick). sure, this will not fade with time and will do much damage, so destroying it is the only option.
since he will be the weaker fo the three listed, he simply dies when HP reaches "0".
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cell.
1º stage
he will keep standing in the middle of the "arena", and will "mirror" the players
if the player do an melee attack, he will respond with an melee sequence, but followed of an stunning and pushing attack that will throw the players away.
if the players to an ranged attack, Cell firt will block the incoming attack ( here will be an 25% chance of reflecting the energy back to the player, 25% of abosrbing and replenishing some of his mana pool,and 10% chance that the attack will break the defence and doing 50% of the ability normal damage to Cell. the other 40% it simply blocks.
If the players take too long to attack cell will spawn some cell juniors to attack the players (2-3 for each player). these ones only do melee attacks.
when cells life drop to 70% he will built up energy and unleash many waves of minor range attacks ( they will be quick but not powerfull) in a conic pattern in direction of the player.
when it's life is down to 50% he will start teleporting around, just to keep harder to get him.
when cell's HP drops to 15% he will built up energy and unleash an VERY POWERFULL kamehameha
2º stage
when the player defeats him for the first time cell will do an wind attack that throws everybody away then built up energy ( an giant KI flame), and grows much in size ( Cells body).
Now it's an new cell but it's HP will like 50% of the first one. and will have an protective energy shield that cuts the damage taken to 35%
at this stage cell will keep flying in direction of any player that moves or try an attack, but everytime Cell moves he losts his shield, and then will need 2 seconds to rebuilt it.
When Cell's life drop to 40% he will stop at the center of the arena and will try to self destruct, he will down his shield and build up KI and every 4 seconds he will unleash an energy wave that damages the players and slow them, if Cell unleashes 5 waves in a row he will suceed and everybody dies.
But every damage that isn't minor ( let's say 8-14 dmg in warcraft 3 melee) will make Cell lost all energy built up and start all over again.
When he is at 5% HP he will do an last resort DAMN POWERFULL KAMEHAMEHA
if the players manage to kill him while he attacks Ok, if not he will keep performing kamehamehas similat to the one he make when was at 15% HP in first stage.
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Buu
his fight is considering Buu in it's final form ( that "child" like one)
buu will keep shooting homing energy balls at the players until them first hit him.
then he will teleport to another place and begin throwing MANY incridibly fast attacks in bursts straight foward ( i suggest dragonhawk rider projectiles), every time Buu is hit the teleports to another place.
when Buu's HP drops to 80% he will teleport to the center of the arena and do an Starfall like attack ( in fact using starfall with diferente graphics is fine here).
Then he will start to fly around and do both burst shots and homing balls attacks at random, but when he reaches 60%,55%,50%and45% of his HP he will teleport near of the player and perform an lightining like attack that slows him down and drain some HP, so if the players don't work well they will never pass this part.
When Buu's HP drops to 30% he will split in 8 minor versions of himself, that will fly at random and attack players that come too near, each time one copy is destroyed it will split in 4 pink bubbles, if the players don't destroy these 4 bubbles the copy will revive,but this will be more tricky :
the real Buu is hidden somewhere, and will be really damaged only when an copy is fully destroyed, it is the 4 bubbles are gone.
when all the copies are destroyed, an big bubble will apear and the true Buu will come back with 25% of it's HP. Now Buu will keep flying around doing the burst shots again AND spawning copies ( max of 6 at same time) then the process will be the same, except that killing the copies nomore damages Buu
every 30 seconds if is necessay Buu replenishes the lacking copies and heals the surviving ones.
When Buu's HP drops to 10% the copies will fly in suicide attacks towards the players.
Then Buu will hide in the ground but the place where it enters don't heve to be the place from wich he comes out( place some rubble around to players have some idea of where he is).
Now Buu will keep hidden until his HP is back to 15%, and then come out and attack with homing balls.
everytime his HP is at 10% he hides again. So players must destroy the rubble around the cenario ( make sure only at this ponit the players can do this to avoid them to do this earlier).
When Buu's HP reachs 5% he will start to throw many fast splash damage attacks in parabolic curve, ( big energy balls with bigger splash area).
when Buu's is "defeated" he will keep exact 1 HP and do an last attempt unleashing many beam like attacks. And will evade and teleport away from melee attacks and absorb any ranged damage except some REALLY POWERFULL attack, just to force players to end this fight with some epic attack. ( it would be nice an Goku and Vegetta players trying to avoid Buu's attacks and then both unleashing an full scale Kamehameha/final flash againt Buu).
If you need more information then I'd prefer for it to be someone who is well known/trusted on this site and would sincerely like to help me. 
I +repped you. Would you mind if I talked to you more through PM's or instant messenger if you have 1?
