Secondary skill.

Well, tactics really help against shooters and, just as importantly, neutralize the opponent's tactics.

You absolutely must have Earth. The rest depends on what magic is available and what the goal is. Water works as a neutralizer for the opponent's magic; Fire has its own things like curses or berserk/rage. Air is needed if there is a large template and you can use 2 Dimension Door.

Well, there isn't much point in leveling a Necromancer on Jebus Cross, unless it's a good Necro with good skills, but you can't really collect many skeletons on Jebus since there are few mobs; however, on other templates, a Necro with Conflux is very strong.

Generally, it's very rare to manage to assemble the full set of necessary skills; sometimes they throw in all sorts of junk.