DocMoriartty said:
I disagree, mostly because once you lose DR you guanruntee that every hit is going to do damage.
So if 100 archers fire then 5 on average will hit and do damage. 5D8 per round with the ocassional critical is going to run through the hitpoints of a balor pretty fast.
Assuming you have a large enough area where 100 archers can attack, what stops the Balor from flying up “behind the Beholder“ or to any area the ray is not targeting (many elements of the game does not have facing, but a ray is not one of them), teleporting away, summoning in a bunch more Tanar’ri, and teleporting back and wiping out the archers?
The cone merely has a range of 150 feet, so worse case scenario is that the Balor flies away 360 feet on round one (forcing many of the archers to be at even more minuses to hit) and teleports away on the next round.
DR is just one of many defenses that a Balor has.
Let’s look at AC. AC 30 means that in order to have more than a 5% chance of hitting, the archers have to be +11 or more to hit. Typically, you are talking about 7th or higher level archers here. And, one hundred of them. With lower level archers, 5 archers will hit per round for an average of less then 20 hit points per round (100 attacks per round where 5% hit). So, you minimally need two attacks per round or more to do any real damage in a single round. I can guarantee that any Balor that I run would easily avoid the ray, and wipe up on any such tactics.
If you are going to throw 100 archers at a Balor, you might as well forget the Beholder and just make it 100 5th level archers with +5 arrows. Your chances to hit and the amount of damage per hit would be a lot more.
You cannot just look at DR and say “Hey, 100 archers could take this guy out.” Unless the Beholder wins initiative and all of the archers go next, and they do 110 hit points basically in a single round, the Balor should typically escape and win.
Balors do have an intelligence of 20 after all. I assume that the Anti-Magic ray does not lower its intelligence, does it?
But, even if you had 650 1st level archers and one Beholder and won the combat in a surprise round, it wouldn’t matter. An ambush is an ambush. You can make up any ambush you want and key it towards the defenses of the opponent. For example, 300 1st level archers with one +5 arrow each should on average win. Will that scenario actually occur in my game? Well, it’s about as likely as 650 1st level archers and a Beholder.
Quite frankly, unless you create bizarre meta-gaming ambush situations, Beholders are jokes compared to Balors. Yes, the Anti-Magic Ray could point out a potential weakness for a single round, but that’s all it does. If you let 650 archers and a Beholder do that tactic, then you really are meta-gaming anyway since the Beholder and his allies would not really KNOW that a Balor has 110 hit points and that it would take 650 archers to take him out.
In any reasonable ambush (i.e. the Beholder and friends do not know exactly what a Balor can do and how many hit points it has), the archer trick would typically fail. Instead, hitting the Balor with a higher level group of attackers with the Beholder using a Readied action to Anti-Magic Ray any offensive spell casting by the Balor is more “realistic”. Even so, a Balor should typically win if the attackers are not powerful enough to win straight up without the Beholder if the DM is running him intelligently.
only 10 of the "Guys" hit with a 20 on their first attacks, and only 5 roll the +10 to do the crit damage. Now they are doing a whoppin total of 2d8+8 = 24 damage, 28 if they grip the Longsword with both hands. DR 30/+3 Balor Laughs 
The same gets applied for each of the iterative attacks. 
Balor Laughs Again 
