Think of meso guard like magic guard but instead of taking away from mp it takes away from your mesos.[/quote]
Basically this. If I remember correctly (haven't bothered checking description since pre-BB) it halves the damage you take, and 61% of the halved damaged is deduced directly from your mesos (if you're at 0 mesos, you can't go negative, but mg's effect will cease to work). It actually saves mesos.