Phase Shift is a Ship Achievement for the Stealth Cruiser.
"In the Stealth Cruiser, avoid 9 points of damage during a single cloak."
If a player can get there, in the Last Stand, this achievement can be easily earned while fighting the Rebel Flagship's third form by simply cloaking during its Power Surge attack, which should deal out more than nine damage.
It is also possible to unlock in an asteroid field, if the player can stay cloaked long enough.
Alternatively you are looking for a ship with Flak Gun II along with another weapon that deals 2 damage.