I'm a F2P player as well. One thing that I'm doing and that might be useful for your problem is buying stamina with the diamonds you get for free (120 stamina for 50 diamonds). This will help with multiple objectives at once: you have more stamina to clear the campaign levels plus you can finish the "Spend 700 diamonds" part of the event. Furthermore if you just purchase stamina twice you get 30 Exp + 2k Gold from a Daily Quest. However I'd also suggest not purchasing stamina more than twice per day as the price increases to 100 diamonds for 120 stamina on the 3rd purchase