A rather bad idea to aggressively side/forward slip a large jet aeroplane, with pod mounted engines on the wings.
Besides initiating rather high rates of descent there have been cases in the past where aggressive forward slips have resulted in engine pylon damage...as in, slightly bent.
Engine pylons are not designed for this type of maneuver, just as aggressive use of a powered rudder at higher speeds has resulted in....bad news.
If I, as a TRE/IRE, noticed an aggressive slip being used, the line check would be...failed, and the concerned pilot sent packing.
These types of maneuvers have NO business being used on a large heavy jet transport.
Period.