Maybe if you had mages/sorcerors/priests casting major elemental protection after the first mage encounter, instead of the usual strength & protection for evil & magic weapon, it might help.
Each quest needs different tactics, and i rarely see groups actually doing that. It's not metagaming about what creatures you know are in the quest, it's actually using your scouts/priests/wizards in helpfull ways to anticipate what trouble there is and react accordingly. The ogre quest is one of these quests where you are actually told what creatures to expect. You can prepare accordingly and remain entirely IC. That's something you're not supposed to do in Clowns, for example.
This quest is tough because the team is up against both hard hitting foes and magic using foes. Don't expect to run through it like in non magic using creatures. (Trolls for exemple, whose magic is pretty useless). You certainly will need magic to fight magic.
I've done this quest countless times, with big groups and small groups. The only times peopled died to non DM possessed mages were when they didn't to stop to check if the elemental protection where still active. And when the scout reported a group of mages, we send in summons to take the blasts. Ogre mages are like any mages: they die quickly in close combat, their spells can be disrupted, they rely on invisibility, and try to run out of a fight. Those are huge weaknesses.
Moreover, when i read someone saying that his group is "dragging along useless members", i can't help but doubt that. No PC is useless, unless others don't give them the opportunity to take on their role in a group. It's nice to take along lower level characters to give them opportunity to play and progress, but don't think of them as just tagging along: that's a waste of ressources. Low level "whimps" can give some elemental protection, detect invisibility, cast protective darkness, or do the enchantments so that higher level casters can prepare better spells. Whatsmore, Ogre quest min level is 4, and all classes are usefull at that level.
Last, you can't just hope that the items you need in a quest will drop off the first creatures you slay. Spellcasters are here to supply these things through spells.
Communication is the key, both in the quests, and before the quests, especially for casters who can prepare a whole range of spells if they discuss it beforehand. Only sorcerors and bards can't change spells, so talk to them first, and ask wizards and priest to ajust if it's ok with their personnality.