I a pretty sure there is no question Valkyries are the most worthless troop in COC.
Playing a fair bit of boom beach lately and I think a simple fix and a new twist would be make them like warriors in boom beach (they kind of look like similar types of troops). Having them regen hp with each attack would not only make them more useful but would add something a bit different to attacks.
At first I thought the training time needed to come down but as mass Valkyries may be a bit powerful after this change the training time is probably ok so it’s not an army someone can constantly raid with and end up with constant nerfs like hogs.
For those who don't play warriors in boom beach are a fast Aztec looking character who has a giant hammer. They are a very fast melee troop that regens hp each time they hit a target.

