Just finished this, very good, but not great film. I'd put it in the 7.5 - 8/10 range. Front end is too rushed and played for too much comic value(would've been better if the build up was similar to City of God or Sometimes in April, gradual with more of an undertone of menace). The film gets much better in the second half or when he is firmly in child soldier mode.
My biggest gripe is with the costume design, and the overly stylized nature of the film. You know you're watching a film and it seldom leaves your mind. The extras look like someone grabbed a bunch of African male models, and said "You guys want to play hipsters, playing child soldiers?". Everyone's too clean, well fed, and content looking for being in a war zone. Some also look like they've been eating anavar sandwiches

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Elba and the main 3 other characters are all very good, minus the costuming.