Save-or-die trumps good planning...As far as planning and skill is concerned no matter how dangerous or swingy combat gets, good planning can always remain useful. In fact , the deadlier combat actually is the more planning matters.
In 3.5, at level 16 you are ambushed, at night from 400' range 32 level 6 wizards casting fireball. That is about 32*6d6 = 32*6*3.5 = 672 average damage, or 336 if you save for half. The encounter is EL16 so an appropriate challenge. The whole party will probably die in the surprise-round...
At lower levels it doesn't have to be this bad either. I had a party that was ambushed by a level 10 wizard in 3.0 who had cast haste before attacking. He fireballed the party twice in the first round and we already had 2 characters dying. He could have finished us of, but the DM saw where this was going and had him move on.
This is typical 3.x encounters that are totally broken because the system is so swingy. No amount of good planning keeps you away from ALL ambushes, and these ones are really bad. Planning can help but it isn't the solution to a swingy system.