After the Dark Barracks are raised to level 3, which necessitates Town Hall level 8, the Valkyrie troop becomes available.
Strategy
Offensive:
If there are a reasonable number of Valkyries, the Valkyrie can quickly take down any exposed hero, which simplifies and improves the effectiveness of the entire attack.
When attacking, Valkyries must move between two or more buildings and deal damage to each one at once. If the structures are not close to one another, however, beware of hidden traps.
Defensive:
Having to deal with Valkyries can be difficult because they are highly strong soldiers who deal a lot of damage and have a sizable amount of health (albeit not quite as much as a Golem, Dragon, or P.E.K.K.A.
The Valkyries are excellent Clan Castle soldiers. They are especially efficient against large numbers of lightly-armored, melee-range forces like goblins and barbarians due to their splash damage, but they are also incredibly powerful against groups of ranged troops like wizards and archers.
Place traps around structures as close to each other as you can to deal damage or get rid of them because the Valkyrie strikes buildings that are close to each other, like a barracks next to another one.