My weapon is my ally

Who needs allies to flank?

You put your long spear in your opponents foot which makes it impossible for him to move until he (or someone adjacent) spends a move action to pull it out which requires him to succeed a strength-based saving throw. Then you position yourself behind the enemy and can equip another weapon and shield and you also can make an attack versus your enemy. The opponent count as flanked until he is able to move again.