BD can sneak up on teams because they actually two good opening bowlers now... It is not just about all their left arm spinners and Mahmadullah... Can see this being a very close game and with BD going in as the underdogs and nothing to lose, anything can happen.
The whole 'left-arm spin army' thing is just a myth now anyways.
It's only Shakib really. Taijul and Sunny have both looked unthreatening and prone to losing their lengths, Mominul isn't in the XI, and Razzak's done for.
What's been BD's strength this WC is, as you said, the pairing of Rubel and Mortaza, supported ably by Shakib and the batsmen. Taskin's been pretty decent as well.
Nasir, Mahmadullah, Soumya and Sabbir are enough to get ten overs out of. They aren't going to be worse than Taijul and Sunny. Realistically, if the three quicks and Shakib don't send India's top order back early, then it won't matter who is filling in the 5th bowler overs, they're gonna get plundered anyways. Should stick with the balance they had against NZ, with batting down till 8 and Mortaza at 9. Set themselves up to either score or chase around 300.
Ideally bat first if they win the toss, and hope to induce a choke during the runchase.
Highly, highly unlikely they are going to win either way though. India just have too much quality.