The zombie is one of the toughest common monsters in the game. It has good damage output, very high hit points, and it's Undead and therefore immune to poison. This means there's no easy way too approach a zombie. The Priest obviously has a distinct advantage against undead enemies, but otherwise this is just a powerful and threatening foe.
Due to its high hit points, direct damage spells and most forms of "burst" damage are ineffective. High physical resistance coupled with some good attrition magic, such as HALPMEH or BYSSEPS can be a somewhat more effective approach, but for characters without significant physical resistance or damage reduction this slugfest will be difficult and exhaust your resources. Zombies are generally NOT a good choice when going for higher level kills.
The zombie boss, Frank, takes everything that makes a zombie a difficult opponent and turns it up a notch. Only characters with an obscenely effective physical attrition strategy will have an easy time with Frank. For everyone else, get ready for a long haul with this boss.