There is my opinion about the parties and supporting characters!
A party needs:

Big Party:

2 Rangers: for backline
1 Chanter: for put spells on the fighters, despells the enemies and cast mantra to protect the party
1 Cleric: for healing the fighters
2 Sorcerors: as the attack force
2 Templars: as the protectors of the party and the frontline soldiers
1 Gladiator: as the main attacker


1st Lonely Party:

1 Spiritmaster: as the main attack force
(Spiritmaster will have his pet too)
1 Templar: as the protector of the Spiritmaster


2nd Lonely Party:

1 Gladiator: as the main attack force
(at this point you can stay as Gladiator alone without support)
1 Templar: as the protector of the main attack force
or
1 Chanter: for put spells on the fighters, despells the enemies and cast mantra to protect the Gladiator


3rd Lonely Party:

1 Assasin: the main attacker
(i think, if you have train your assasin well, you don't need any support or help, but if you don't wanna be alone then you can find one chanter or cleric for support)
1 Ranger: the backline support
(with these 2 you can make traps, but only if you are trained well)

If you have any suggestion about any new combination, then lets discuss it here!

Big party compilation looks good.. if everybody does what he have to do.

Lonely party... mmh i'm (sorc) playing with a friend( templer)and it works.. tank + mass dmg

I heard spiritmasters have a hard time finding partys is this true?

Every class is hard to find party, thats why is good to play with friends so everyone picks his class and help each other!