We (7Characters at 13th level) had an encounter with 3 Wizards in the plains. Open sky ! First they used enlarged Fireballs, then they hasted, casted improved invisibility and next round fly. Later Stoneskin and some other protections.
They took us out like sitting ducks. No chance to find them. We didn't know where they were so it was hard to ready spells on them. Most of the time they were out of range for See Invisibility. Faerie fire caught one of them but only one ! Close Combat Body Guards kept the fighters busy. One mage concentrated on counterspelling our mage. One mage threw out Summon Monster and Wall Of Force.
We were doomed but luckily the great red wyrm decided to intervene (we called him before the battle started). So we survived ... this time.
Another example: Hasted, Flying, Imp. Invis. Wizard with Maximized, Quickened and Empowered Magic Missile and a second Empowered Maximized Magic Missile in the surprise round. Took out our wizard in a second. Next round he won Initiative and blasted the second mage with similar spells and a fireball took the rest of the group in dire straits.
He used no special or fancy spells, just the ones described and killed our group alone, without support. OK, we survived because it was a special kind of dream, but in reality (D&D-reality) we couldn't survive one wizard.
Hm, that seems no fun but it's unreasonable for a DM not to play a wizard as effective as he could. Remember the high INT of the wizards in higher levels. No way to play a wizard stupid.
Just a little comment on magic in D&D ...
By the way, I play a fighter type, so I can blame all wizards ...

Next time I play a wizard I will blame the fighters for unrealistic high damage every round ....
Bye