The disease line of dots are near useless as it takes forever to do actual damage, so it isn't that important to invest in disease resist or counteract disease while fighting a necro. Stay on the necro while fighting him, use your bow to keep him on target and to interrupt spells, same with all casting classes. The hard part of fighting a necro is his life taps, if you cannot interrupt them, you cannot win. They are ALL magic based. If you cannot be mezed, stunned, or nuked, you cannot be killed by this class.
Tashani series is poison based, but is near impossible to resist.
Use counteract poison to remove tashani series. Interrupt his gate spell and with a high MR, will be a easy kill. Use a pumice stone to remove all buffs and to remove all debuffs that stick on you. They are just annoying, songs are unpumiceable, and can easily kite a warrior. Use your bow to wear down the bard if he tries kiting you, or just run out of song range and wait for him to come back.