What to do with big stacks:
1. Use HQ, for double you commander limit
2. Attack from so many directions, as you can. Don't forget about amphibious assault and paratroopers.
3. Use aviation. Destroying infrastructure decrease ESE, interdiction decrease organization, bombing decrease strength. You should start from Strategic. in this case org and str will restore slower.
4. Use shore bombarding. Your opponent could get up to -25% penalty.
5. Use the hardest units in narrow front. Armours with H-Arm or even SH-Arm work perfectly in this case.
6. Use waves. Attack, lose org, move back, rest, another attacks, etc.
7. Use engineers. If attack, they destroy opponent's infrastructure with high rate. So, engineers should be in first wave, and better on tanks, for longer period.
8. Encircle. Encircle one province, or even five.
9. Cut from supplies. Some times, it is easier, than encircle the big stack.
10. Use big stacks despite on penalty. In this case, use Militia totally as canon-fodder. For example, attack with general with 9Armors+21Militia. Armors will not suffer from penalty, and militia will attract 2/3 of shots.
11. Don't forget combine bonus.
12. Remember terrain and weather bonus too! Check for rain, frozen and muddy!
13. Use offensive supply. it will give you +10-20%
14. Don't forget about Artillery brigades. Big stacks is the case, where you need them.
15. Think, why you should deal with big stack? If so much units in this segment of front, then another segment should be somewhere, completely undefended.