Just simply install it via pacman -U mingw-w64-x86_64-qtbase6-static-6.4.2-1-x86_64.pkg.tar.zst.
This version uses Qt-bundled md4c to reduce dependencies.
NOTICE: Built from community edition. Make sure that you obey Qt's GPL/LGPL license. Do not use it on proprietary projects, or closed-source commercial projects.