I'm going to guess you're in Icirrus City right now?
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Yamask (later Cofa grigus) is an excellent defensive Pokemon, not very strong but he he great at what he does. A good choice, and one I personally haven't done yet. He's slow, but that is not a major problem for him because he is a defensive Pokemon.
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Solosis (later Reuniclus) is rather difficult to use, he has amazing HP and Sp. Atk and decent everything else except Speed. His speed is rock-bottom, and he is only able to outspeed people who are even worse (such as Gigalith). The best sets I've read for him are only doable after the Elite Four, such as using Calm Mind and Trick Room (those TMs are obtained after the Elite Four). My advice is to drop him and wait until after the Elite Four to train him.
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Tranquill is a Pokemon who looks promising, but in the end doesn't have the stats to fight very well. My advice is to drop him.
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Gurdurr (later Conkeldurr) are a nice addition to your team, his movepool basically only has Fighting and Rock moves but in the end, this guy is an absolute powerhouse and you will definitely want him on your team. If you can get him to evolve, that is. If you don't have anyone to trade with, I would suggest dropping him. My brother tried to train a Gurdurr without evolving and it just didn't work. Also, he isn't very fast, but he isn't really slow either.
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Boldore (later Gigalith) is a pretty impressive Pokemon. He has huge amounts of Attack and Defense, but his special side is very low and his Speed is literally the third slowest out of all fully evolved Pokemon in Gen V. That's even slower than Reuniclus. It's really up to you whether or not to train him, if you don't think his special weakness will be too limiting then go ahead.
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A great choice. Samurott is a fairly Powerful Pokemon who doesn't lack defenses either. His Sp. Atk is slightly higher than his Attack, but you can still use physical moves regardless as they are both high. I really don't have much to say about this guy.
My advice is to certainly drop Tranquill and possibly Boldore and Gurdurr (only if you can't evolve them). I also would caution against training six Pokemon at a time as the Elite Four are not very weak this gen. They aren't ridiculously strong though and it is possible to beat them, but very difficult. I only trained four Pokemon: Excadrill, Reuniclus, Conkeldurr, and Larvesta. You can train five though, I've done it before.
If you have to drop Gurdurr and Boldore, I would suggest replacing them with Joltik (later Galvantula), Klink (later Klinklang), Drilbur (later Excadrill), Sandile (later Krookodile), or Vanillite (later Vanilluxe).