We will simplify this expression right to left:
#81^(16^(-4^(-2^-1))) + (4^8)^-3 * color(blue)((4^-6)^-6) =>#
#81^(16^(-4^(-2^-1))) + (4^8)^-3 * color(blue)((4^(-6 * -6)) =>#
#81^(16^(-4^(-2^-1))) + (4^8)^-3 * color(blue)(4^36) =>#
#81^(16^(-4^(-2^-1))) + color(red)((4^8)^-3) * color(blue)(4^36) =>#
#81^(16^(-4^(-2^-1))) + color(red)(4^(8 * -3) * color(blue)(4^36) =>#
#81^(16^(-4^(-2^-1))) + color(red)(4^-24) * color(blue)(4^36) =>#
#81^(16^(-4^(-2^-1))) + color(red)(1/4^24) * color(blue)(4^36) =>#
#81^(16^(-4^(-2^-1))) + color(blue)(4^36)/color(red)(4^24) =>#
#81^(16^(-4^(-2^-1))) + 4^(color(blue)(36)- color(red)(24)) =>#
#81^(16^(-4^(-2^-1))) + 4^12 =>#
#81^(16^(-4^color(red)(-2^-1))) + 4^12 =>#
#81^(16^color(red)(-4^2)) + 4^12 =>#
#81^(16^color(red)(-8)) + 4^12 =>#
#81^-128 + 4^12 =>#
#1/81^128 + 4^12#