It was quite quite easy. Split up your 66 stack into quarters (avoid attrition through empire), walk through HRE, combine stacks in your provinces, pick apart one stack of Flanders at a time. The AI, most of the time, isn't smart enough to stop sieging and engage you in the battle. They have about 88k (four 22k stacks?), but you have tactics.
Take Antwerp and Hainaut in one war, Vlandereen and whatever province is left in the second.