This card is a must for any undead deck, but if you play with it, you'll probably experience some feelings of guilt because this rule card is almost unfair. Since base level is subtracted from base level, your opponent cannot load up his champions in the pool with artifacts and magic items to help them survive clashes with your undead. Even low level undead are to be feared with this card in play since even if they don't outright kill an opponent by reducing his level below zero, they still probably have at least halved the level of their opponent.

Negative Planar Energy does not activate until after a defender is brought forward. If an attacking champion, such as the Living Wall (58/PO), would instantly defeat the defender before the defender's power activates then this rule card doesn't activate.

Strategy then should be to use high level undead champions like Nemon Hotep (67/NS), Winslow the Lich (69/NS), or Caller in Darkness (70/NS). This can eat up your champion level total when building your deck, though, so look for champions who gain levels (before combat, not during) like The Undead Htimsen (425/3rd).

Another good strategy is to play with cards that create undead like Corruption of the Flesh (86/NS) which even adds four levels (although it is adjusted level, not base level for the rule card), Black Snail of Shnai (35/UD), or the dungeon card Under Castle Strahd (7/DU). Imagine champions like the Living Wall (58/PO) draining levels. Also, cards played before battle that affect base levels can add interesting twists to this rule card - playing Bad Omens (91/3rd) when your undead are immune to harmful events, for instance.