Switchers are purple-tier Eagle Tribe soldiers on Iki Island who carry several weapons and swap between them mid-fight. You have to keep changing stance to match whatever they are holding.
A switcher changes weapon during the fight, so a stance that worked a moment ago may stop working. With the shield out he blocks like a shieldman; with twin sabers he combos like a swordsman; with the halberd he sweeps like a spearman.
Watch the weapon, not the man. Swap to Water for the shield, Stone for the sabers and Wind for the halberd. If a shaman is nearby, kill the shaman first so the switcher isn't buffed as well.
Eagle Tribe
The Eagle Tribe is the Mongol force that has taken Iki Island, led by the shaman Ankhsar Khatun, called the Eagle. They are the main enemy of the Iki Island expansion, and their purple-uniformed troops bring units you never see on Tsushima.
Shamans
Shamans are Eagle Tribe support units on Iki Island who stay at the back and chant to power up nearby Mongols. Buffed soldiers get double health and turn far more aggressive, so the shaman is your first target.