I noticed that WoW often negates the purpose of achievements, specifically raid achievements.
Being overlevel can make things a lot easier, but some achievements are still tricky and should be if it can be helped. Now if killing a boss in a certain timeframe is the achievement, there is little you can do against overlevel soloing this without any problem. But let me give you an example of what surprised me a bit.
For the Lich King there is an achievement where in one phase of the boss fight spirits or such appear and you need to mske sure none of them explodes. Now when you are soloing the encounter with an overlevel char, you might kill the boss before that phase has a chance of appearing. Now the proper strategy for this would simply be to not nuke the boss down but let the phase occur.
Instead what happens is that your achievement panel announces to everybody that you successfully prevented spirits from exploding in that encounter when you actually simply nuked the boss down. You get the achievement because it never even had a chance of failing.
Same earlier in the same raid instance when you had to heal a dragon up to full health. There's an achievement about entering portals, and if you heal her up quickly, those portals don't appear and you will get the achievement for entering them.
I find that a bit lame and don't understand what the reason is. It's easy to cut back on your damage or heal output, even without removing gear to weaken yourself.