Although this is a forum for magick, your purposes may be best served by combining it with other efforts. In that spirit, I am submitting a scenario for your consideration . Use it in whole, in part, or dismiss it altogether, as you like.
Magick would serve best to get the ball rolling in two ways:
First, free yourself from animosity that might betray you. Cease even the appearance of hostility against them. Devote yourself to the duties left you by your boss, and show him your resolve to excel, whatever your assignment. These elements will be essential later.
Second, get the two in discord with one another. They are bullies, and will both seek alliance with the boss against one another (as they did against you). The boss may not wish to take sides, but he will ultimately have to, when faced with either loosing the loyalty of one, or of both. This will create one stronger (allied with the boss) and one weaker combatant(on which you may fruitfully focus your attention).
This is where the work truly begins:
Bullies cannot operate alone; they need the security of allies. The weaker will be driven to seek new support. By showing no animosity, you enable him approach you. He will attempt to cajole you by shifting all blame for the previous conflict onto the other, perhaps apologizing for a far more innocuous transgression (ex. Being talked/deceived into it). Let him believe he has succeeded. Do not, however, join in the fight yourself, nor provide him any substantiable claim that you are in league with him. You are merely to feed and fan both fear and fury. You may well seize opportunity to have him exonerate you with the boss, under your apologetic assertion that his outstanding deception renders you incapable of any influence.
Know, before you even commence, that he will lose this fight. Continue yet to commiserate, and subtly nurture the notion of vengeance against both foe and boss. As before, even the unsecured hope of your support may drive him to divulge the boss’s complicity the plot against you. Once he betrays them, as his ego should dictate, by going above their head, whistle blowing, etc., the boss will see him as a threat and the other as a liability (especially if their prior collusion is openly exposed).
All the while, show your boss that you are focussed on your work, and are remaining distant from the fray. The boss now values you, needing someone upon whom he can rely for consistent performance, to resume your previous functions, and perhaps more. His entanglement in your defamation may cause him also to fear your testimony to superiors, and may inspire him to appease you. It may also serve to insulate you from future slander.
This submission for your consideration stems from a similar situation of my own, with which I am preparing to employ magick. A former manager is enraged at my recent change in departments. Unable to punish me or the boss who now benefits from my services, as she had promised to do, she has lashed out a mutual friend and co-worker because of our relationship. Furthermore, she has taken the opportunity to exploit the ignorance of her insecure new boss to complicate the operations of my new section. As the differences between our situations would suggest, my proposed course is unidentical to that described above, but the partnership between magick and mundane effort is similar in nature.
Hope it may be of assistance,