/summon Skeleton ~ ~1 ~ {Attributes:[{Name:"generic.maxHealth",Base:200}]}
That should summon a skeleton with 200 health but whenever i summon this or any other mob with any custom amount of health its will always spawn with its normal amount of health so we cant create mobs that should survive a lot longer then normal for custom maps
You need to specify it's current health as well. See the linked report.