Crusades are bad, you need to occupy too much to sue for peace, create a decent sized retinue/mercenary army (i usually go 10k) holy war for a duchy and occupy the duchy before they can organize their small armies into one big one, remember to fast siege it, assault it, remember, you need at least a force 10 times larger than the garrison to win the assault with light casualties (usually 10k is enough) then you should be able to sue for peace (if the duchy you want is all under enemy rule, if they have only half of the duchy then occupying it won't give you enough warscore, in that case destroy its divided armies and occupy the enemy capital)