You also have to consider what you have for donors. I have a full pally donor set WITH churga, and I can assure you, any full donor mage will easily take me down at lvl 400... for a pally to survive any single full donor mage (even lvl 300) he must be 500+.
Also think about this, if he is healing with a churga (40 pt) and you are attacking with Double Overtime (5pt) clearly he should have bought that battle if you know what I mean. Also if you are using double overtime, im assuming you dont have Cyntarist wand... makes a huge difference considering the other wand does like 500 dmg and cyntarist wand hits for constant 1.5-2k.
There are so many variables in the battle that it is possible if you aren't full donor you could have trouble with a 400 pally who IS full donor. On the otherhand if you were full donor mage, you would no doubt own a 400 pally unless he spams pots, I promise.