Posted on 23 December 15 at 19:17
Being the driver on the Fleeca heist missions is really the only guarantee. Just host the heist, and in the first mission make it to the driver's seat of Lester's car before your partner can, and in the second mission be the one to drive the Kuruma back to the factory. Then when you get to the finale, assign the Driver role to yourself. You'll get 3 platinums per run of the heist, and the only problem you have to deal with is randoms who don't know how to hack/drill.
I don't know how the other heists award medals, but it's a crapshoot. Just grind your 8 million in other heists, then come back and grind Fleeca for medals if you don't already have 25 by then.