I think you guys are really looking at that the wrong way.

Instead of thinking "Blackstar is weak because he couldn't even damage mosquito with shadow star" I think it's more like "Mosquito wasn't kidding when he said that form was when he was the toughest since Shadow star didn't even scratch him".

I also think the reason that we haven't seen kid doing any training other than team Resonance is because he has already done that those things? The story has been rather vague on the difference between a Shinigami and a regular technician but i'm more incline to believe that a Shinigami develop his power as he grow older rather than through some sort of training.

Recently most of the opponents they are fighting again are on a totally different level. Mifune was able to fight Cid on even ground and Cid is suppose to be one of the highest rank technician.

I'm not too sure that i can discuss manga content without it being spoiler but the conclusion to the fight with Mosquito was different in the manga.