For reasons unknown, Zarevok seems to be able to hit monsters harder than one might think. For those of you who don't know, Zarevok is a somewhat weak general, who offers +20 Attack when you use him to fight monsters. What is interesting is that he can sometimes do more damage than Generals with better stats than he does.
Compare Zarevok to Vanquish. They both offer +20 Attack. However, Vanquish's natural Attack and Defense are much higher than Zarevok's (at the cost of having a -25 defense penalty). You would think that Vanquish's natural Attack (18) would make her a better choice to use than Zarevok (who has an attack of 6). If you thought this, you would be wrong.
This has spawned a whole slew of rumors.
Does Defense matter when fighting monsters? No, Only Raw Attack Matters.
Do some Generals offer a special bonus against certain monsters? This would explain why Zarevok is great against some monsters, but is horrible against others. However, there seems to be no rhyme or reason to why certain Generals are better than others. The small character biographies that the Heroes do have do not seem to affect their damage output against monsters. Their personal history, their use in quests (both main quests and Demi-quests) don't seem to have an effect of why they make a difference. It's not their natural Stats, or even their special bonus that makes the difference, so what is it? How does this mechanic work?
Unfortunately, no one knows at this time. Before people could really delve into this and try to find out what this is, the Vanguard Chest was released, and in it, Malekus (Hero) and Lotus Ravenmoore (Hero). Malekus has the potential to have the raw attack power to be better than every other General for attacking monsters. Lotus, on the other hand, is currently better than every other Hero. She offers a +4% chance to get a critical hit on a monster, which in the long run, is better than getting an extra 50-200 damage on every hit from the other Generals.
So, this debate is mostly moot. If you are a new player and don't have many favor points, be sure to try out different Heroes for each monster, and remember that Vanquish and Zarevok are not necessarily the best for every monster that you will fight. Example: the demon General Araxis will do consistently more damage against his former master, Mephistopheles (regular, not Alpha), than both Zarevok and Vanquish. However, if you can afford the 30 favor points, you should definitely buy Dolomar, whose +5% chance to get a critical hit bonus makes him clearly better to use against every monster, or if you have Malekus and have 100 other Heroes to power him up.
Zarevok's Bonus Against Monsters
Zarevok is known to be especially potent vs. the non-Bahamut dragons and some boss monsters. This, plus his bonus, makes him likely to be your best monster-fighting option until you get Vanquish, or get lucky with an epic chest General. For most team and world monsters, you will find Vanquish and other heavy hitters are more effective than Zarevok. The wise monster fighter tests his or her available monster-fighters against each kind of monster before assuming "General X is best." Dragons were the first non-boss monsters and use a different damage formula than the others. Clearly, Zarevok was given special consideration for dragons and it follows that some earlier bosses may treat him specially as well. However, the reasons why any of this is, and the general mechanics behind the Hero's influence on damaging monsters, is unknown.
A useful reference for the performance of general against several common monsters is given on the Castle Age Forums. In particular, see posts 7, 8, and 9 of: 'Generals - Special Ability Database', or the graphs found here: 'Does adding Defence on status helps attacking damage as well?'.