I'll start with a disclaimer that I don't know all the pokemon but I've seen a few so I feel like this fits the bill! I think the concept is quite nice, instinctively I think about the turtles in mario for some reason and the fire levels but still in the pokemon universe. Further I love the expression as it almost looks like he's sneezing, which makes the fireballs quite nice since they could then be incidental as well as a weapon. That adds some comedy to the image which I don't know if its intentional or not, but still nice. The fire itself looks awesome and I'd love to know how you've done it?
Further, the perspective, composition and backround of the image are great (Especially background with text about Galapagos) since they make the turtle look huge which I think is the intent? Further I like the subtle cartoony shading under the shell, around the eyes, legs, etc. as well as the smoth color transitions on the front of the turtle and the shell.
But I also have some critique. First I think the shadows and sky are too dark, i.e. black. You should have gone with some darker reds instead. Further, given the Pokemon universe they seem to work a lot with contrast colors to make things pop out. Thus you could have considered a wider colorpalette, perhaps with some greens or blues in the background. The mountains and ground could have worked well in a grayish blue I think and still captured the galapagos environment. I'd also liked to have seen a vague auro of white around the turtle to push it a bit more in the foreground and that the mountains where even a bit paler than they are and perhaps even a bit blurry to really push them backwards. These points motivate my 4/5.
Still, I really like this concept and think it would be a cool pokemon to fight with :) Very cool.