for north/south guardian I used strength build, with 2x flash freeze, vapor, 2x ice wall, subversion, hypothermia, and decimate, keep everyone alive untill you can kill south guardian. Then just keep yourself alive, and kill the remaining guard with combo: flash freeze, decimate, flash freeze, hypothermia, subversion, ice wall, vapor and rinse/repeat. Subversion hit will heal you fully, you can take two hits in ice wall, and he will go down in two or three runs. Watch for focus.
Similar build takes down yosuke very fast.