I was lvl. 37 when I killed th telethia now I am 39And the new girl is kinda useless, or do I not know how to fight with her on my side.

It's worth going over the tutorial for her, because she functions like no other character. Keep in mind that you'll actually get a brief forced tutorial on how to use her a bit later, but it's worth digging through the game's menus to find it now. In essence:

1) Unlike other characters, her gray bar (the name escapes me right now: it's the talent in the center that other characters besides Sharla have to fill up before they can use it) does not have to be maximized to pull off her special move. She can use it anytime she has summoned at least one element.

2) Selecting a summon acts as a buff for the party. I believe these buffs do not stack with each other. For example, summoning Thunder increases everyone's Ether attacks by a certain percentage, depending on how many levels you've pumped into the Summon Thunder talent.

3) She can summon up to three elements at a time. They can be identical, each be unique, or whatever combination floats your boat.

4) As earlier alluded to, using her gray talent art uses these buffs as an offensive weapon: she "throws" the summoned element at the target, which deals ether damage. Obviously, you lose whatever buff that element granted you. This is the key to dealing damage with her!

5) Important thing to note: most enemies have little to no real Ether Defense, so she can tear through foes with high physical defense. I believe these attacks are also impossible for the enemy to dodge.  Nebulae in particular illustrate how valuable she can be to the team.

6) Like all ether arts, she draws lots of aggro when summoning/attacking. Make sure to give her Arts Stealth gems/skills!

So to recap: she can summon elements to buff the party, then use those elements offensively to deal ether damage. It's important to note that the AI has zero clue how to use her, but if you take control of her most fights will end quickly. Her Summon Earth attack deals high, if not the highest DPS in the game, Summon Fire is a nice DPS backup, and Summon Thunder does the most outright damage, plus it recharges ridiculously quickly. Just try to keep at least one Thunder element around, as it boosts her Ether by up to 20%. You can safely ignore Summon Aqua: it's HP absorbtion rate is pathetic, and it does paltry damage.

She is anything BUT useless.  .Melia is, potentially, the best character in the game! Her Arts are awesome, she's incredibly strong, can heal (although she's not a great healer), and can auto-topple an enemy with Starlight Kick! She's also very easy to chain with, as her Talent Art can be used atalmost any time you have an elemental up. Her main role is to use the elementals to increase her stats, and then discharge to deal damage and harm the enemy. Provided you're playing as her, she's almost a mainstay in your party. It's a good idea to practice with her in the next area before things get too tricky!

I'd say closer to 50-60 hours, which is still below average. But yeah, there's soooo much stuff which opens up after a certain story event, including the extended areas of certain areas, dozens of new quests, all the stuff for Colony 6, and all of the 5th skill trees. You've got plenty more to do.

Just try to do all of the timed quests before you head into the abandoned capital. And do all the quests IN the abandoned capital before fighting the next boss. I don't know where you are, but it's general advice. The game doesn't war you, but several areas are locked out at this point. 

Oh, and do EVERYTHING on the sword and fortress as possible, ASAP! 

I´am only playing as Shulk when I use her I use her as Squadmate, right now I´am playing with Shulk Reyne and she are my fighting diversion

I'd ditch Reyn and Shulk, play as the woman and also add Dunban/Riki. Both are far superior, and the next few bosses Shulk can't do any damage. So you may as well try with some other characters for practice

Against bigger bosses I fight with sharla in my team to heal and Reyne or Dunban, but for smaller enemys Reyne Shulk Dunban are the best team cause you can end almost every fight in 10-20 seconds

I disagree. Reyn is absolutely useless now you have the other party members. He has no defence, no speed, no strength (even Riki is stronger, and he has far superior Arts), his Arts are pathetic etc. ALL characters outclass him in every way. Believe me, you want him out. Dunban or Riki make far superior tanks, and Melia and Shulk are far superior damage dealers.

Also, Sharla can't chain well at all. She only has healing Arts, and her strength+ Defence are pathetic. Also, the cooldown is too long, and the AI may heal/protect you at the wrong time, so it's best if you play with her, but if you do, you deal little damage. She's a good healer, but I'd try with other characters. Riki and Shulk have good healing spells for emergenices, for example.

I'd recommend Riki/Dunban/Melia at this stage of the game. Best to control Melia, because Riki's and Dunban's AI are very good. Shulk isn't needed here. Use this time to experiment with other characters, and choose your favourite fighting style for the second, more difficult half of the game.

I´ve tried to mix things up but so far the best team I had for smaller like this giant unkors or Diraups for example were Dunban Reyne Shulk even when I fought against 3 or 4 enemies at the same time I didn´t had any problem to finish em of. But with Rikki and Dunban I had a little Problem with iglers on the bridge to colony 6 but if you say that shulk has no affect on upcoming enemies then I will switch things up, but I don´t like be the healer only in FF XIII it was kinda fun.

btw. My Heropon looks like a damn Badass I´ve gave him some Panther gear and Makna shorts he looks super cool, the girls look like whores tough :/

Yup, the Heropon is pretty badass!

Of course, it's your game, but I think you may find it more difficult playing with Reyn later on. Dunban is a mainstay IMO. He's just got everything going for him. Same with Shulk (but since he's useless in the next couple areas it's best to raise the affinity with some other characters). 

And I'm surprised. Sharla always looks a little bit...under-dressed, but Melia always looks classy. Riki is the biggest whore anyway; mine looks like a tiger. What a pro.