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kwendy is it so hard to use [ spoiler ] when you're spoiling ?
So far the only SUCCESSFUL trainers we know of is the one of the
legendary trio (which is the same guy that inspired the MC).
Then we have MC and the new girl, both seem good but they wouldn't win against a great god/dragon/equivalent level trainer. They still have room for improvement.
The whole clan of slime tamers are considered CURSED, even from this clan's point of view, only the new girl's trying to make this view change by becoming strong (and marrying someone who thinks of slime as she does). Her clan beside her is not strong, they're not "great" tamers.
With all we know, there's only ONE legendary slime trainer. The only good ones we know of are the MC and the new girl. That's it. Written black on white in the manga. Her clan is so bad they call it a cursed clan, other tamers are so bad she didn't even consider them as candidates. You have it : one legendary guy, two good ones, other than that, they're all believed to be utterly weak.
Her clan might actually be better than your average slime trainer but that's it. They developed techniques to try and become a bit stronger but so far, the way it's shown to us is that only her was truly able to use these techniques to such a level.