There has been a lot of discussion on the paladin boards lately about how casting Holy Warrior will mess up their learning. This article is dedicated to showing both why it is messing things up and how to use spells to adjust how you learn defenses.
To understand the issue, you first need to understand the way defenses are applied. To begin with, the attack goes through your evasion, then, if it goes through that, it goes through the higher of either parry or shield, then the lower of the two, before going to armor and stamina. Each part it goes through reduces the damage until you stop it or run out of things to throw at it. If you stop it in the evasion, parry, or shield stages, that is what you learn. Thus, you will want to make sure that you stop the attack at the point of the defense you want to learn. People mostly do this by fiddling with stances, however, spells can do this effectively, as well. Just takes a lot more mana and a little more finesse.
As an example, here is a graph of Benzali's defenses when he first steps into the croc marshes with a 60/60/60 stance setup. Yellow is evasion, cyan is parry, magenta is shield. The red bar shows the range of attacks that crocs can deal out. It is, in truth, simplified considerably, as there are actually areas of overlap.
Note that the attacks fall within the evasion range and largely within the parry range. Thus, I learn a lot of parry skill in this situation, a fair amount of evasion, and little shield. Let's see what happens when I cast Holy Warrior, though.
The increase in effectiveness of all defenses causes a shift in defenses learned toward evasion because my evasion has become so effective that little gets through to my parry and very little reaches my shield. On top of this, you can not adjust your stances to learn the way you would like. This is where spells come in. Your major spell to shift the balance is Divine Armor. Here is a graph of what it looks like with Divine Armor, instead.
Note that I would be learning shield and parry very well like this. Now, combine the two.
This shifts things close to the center without even touching my stances. I also have one of the best defensive spells available active at this time... and Divine Armor. Divine Armor also has the added value of bringing the hinderances of all your armor pieces closer together, balancing out your armor learning, as well.
This isn't completely optimal learning, though. The best learning combonation would have all the defenses perfectly balanced within the range of the critter's attacks. It would look something like this.
However, this would be fairly difficult to do, as the second defense must be higher than the third to put it into this position. Thus, you need to somehow modify your second defense to reduce it enough. If it is parry, you could use a weapon you aren't very good with. For shields, you could use a small one. Just fiddle with thiings until you manage to get it just right.
A far simpler method would be to increase the range of attacks you defend against. The simplest way to do this is Sentinel's Resolve. Just cast it on one of the critters in a multi situation and you will reduce that one's attack while maintaining the same level of attack with the rest.
Other spells that will affect your defenses:
Courage will cause you to learn more evasion if you are burdened.
Heroic Strength will do the same.
Sentinel's Resolve will increase evasion learning and, to a lesser extent, whichever defense is your second.
Righteous Wrath will shift towards the later defenses.