Ok, after I reading and writing a few posts in this forum I got many insights on how to develop a better party and which faults i did at the beginning. my problem is: if I delete a character and make a new one he needs an eternity to keep up with the other ones (at the moment at lvl 8 ).
how can I make this more quickly?

you could just rest in a place that has powerful enemies and battle em until he is high, or you could use cheats.

I am doing it right now. My party was around level 9 when I fired the useless monk and started a rogue. It doesnt take long, just protect the character. Missile weapons only, maybe pick up a fighter level on the way up. A little tender lovin care ;) and they will be in the thick of it soon enough. My partys around 12 with the rogue quickly catching up at around level Rogue5/Fighter2 and doing well.

Okay, I had made some errors regarding party composition, PC developement, blah, blah, blah. I exported 3 of the party members and started a new party.

The newbies are moving up fast.

Simply leave the new character behind in a safe spot. He does not even need to kill something to get xp.

Lets say you just entered a new map and created the character. Leave him at the map's entrace and take your other characters to clean this map. The newbie will get his share of the xp and level pretty fast.