You can win cup on level 1 with 3*-4* players in 1st team because opponents are usually bots. Then you can win again on level 2 if you buy few Q25-29 players + enough Q20-24, with max. 4* average Q. This way you might get not-too-much-strong level 3 cup draw for which you will need a lot o 5* players. Later forget about cup.
I don't have to say that you need player which have good morale, which are in form (7 or higher), good tactic, counter formation for some stronger opponents, luck, red packs to keep best players in 1st team etc.
Once you win cup, you will fell like you won real world cup. Winning cup is the best feeling and archievement in top eleven (at least for me)