Necessary Defensive Spells & Skills For Every Character
There are three main ways to take less damage in combat. These are via armor class (AC), stats which help defense, and defensive spells. This post will focus on the latter, but all three are tied together in more than one way. You need to use some defensive spells to ensure you aren't dying in every fight, and especially so when fighting boss mobs on maps.
The Cleric Spell Armor
This is one of the easiest defensive spells to get, and it will be the first one for many people. If you build your character right, and you have cleric in your first three classes, you'll have this quite soon. It has a low level requirement of level 3 cleric, and only needs 13 wisdom, and 11 charisma.
You'll get +20 in armor class for a cost of just 6 mana per cast, and it lasts for quite a few game hours. In addition to that it can be maintained down the road if you learn the mage spell maintain. It is not dependent on your cleric cast level, no matter what you are going to get 20 extra armor, and it is a perfect start to building up a bare bones armor class.
The Cleric Spell Sanctuary
In my opinion this is the most important defensive spell anyone can learn. It is nearly impossible to play Alter without knowing sanctuary. I've lost count of how many people didn't know to keep their sanc up, and had dying issues. It is crazy how much less damage you will take with a sanctuary up.
Sanctuary is dependent on your cast level, but even a low level sanc, meaning casting from your second or third class is more than enough to make a large difference in damage absorbed. It has a hefty cost however, so it isn't something you can recast until you have the mana regen to support it. Each cast costs 75 mana, and it requires 20 wisdom and 16 charisma. This is high, especially since it is only a level 11 cleric spell, but it is worth the time needed to get the stats in order to cast it.
The Mage Spell Shield
This is the second spell most people learn after the cleric spell armor. It comes in at level 12 mage, so a bit later unless mage is your first or second class. The stats required are 20 intelligence and 16 wisdom, yes this is a quite high intelligence requirement, but it is worth it in the long run.
The cleric spell gives you a +20 armor boost, and the mage spell shield will give you another +30 boost. Together these are about half of the total spell based armor you'll be able to get. Also the spell shield is meant to help prevent projectiles from hitting you, and projectiles can do an abnormally high amount of damage, so the more help in that regard the better. It also has a quite small mana cost of 18, so even a thief or warrior can add it to their recast list, but it cannot be maintained like the cleric spell armor.
The Mage Spell Displacement
This spell will come much later for most, but it is one of the best you'll be able to get. If you are a first or second class mage you'll have it quite soon, and you'll be set up earlier than most. It gets unlocked at level 19 mage, and has a hefty intelligence cost of 24 with 20 wisdom. These stats are an issue, and you can expect to put in some time training up your stats if intelligence and wisdom aren't your classes's main stats (This is why necro mage and druid mage is such a good combo).
The spell displacement gives a really nice armor boost of +40. At this point with all your spells added up you'll have an armor class of 90 even when naked, and that is what the game will be expecting you to have. With the equipment you are wearing you'll more than likely range between 100 and 110 armor class, and that is perfect for any caster, and even for a warrior or thief with high skill. It does have a cost of 30 mana to cast, but it is worth recasting. Also like shield, it cannot be maintained.
The Druid Spell Thorn Armor
This is very class specific since it is not unlocked until level 30 druid, but it is very useful if you have druid as a first or second class. It has a pretty hefty cost of 35 mana, 28 intelligence and 30 wisdom to cast, it not only gives +25 armor, but also 5% normal save. Norm save is something which shouldn't be overlooked, it becomes more important on the mainland, but also helps a lot on the third island. If you can swing thorn armor with all other defensive spells you'll be looking at 105 AC even when naked, and with an extra 5% armor save on top of that. If you can befriend a druid who makes spell staffs, you can get some thorn armor staffs, and get the benefits of the spell without needing to know it.
Mage Elemental Shields
There are three main elemental shields: fire shield, ice shield, and ground, with mana shield as their cousin. Unlike those mentioned above they are not meant to be recasted, but rather used in specific places and situations. Ice shield gives 25% resistance to fire, Fire shield protects against ice damage at 30%, and ground gives 25% resistance to electrical attacks. The first one unlocked is fire shield at level 11 mage, then ground at level 12 mage, then finally ice shield at level 13. They range between 19 intelligence and 15 wisdom to 21 intelligence and 17 wisdom. This just means if you are already working on getting the spell shield and displacement, you can pick them up on the way when you have extra practices.
I should give a passing mention to mana shield. This has very limited use, but where it is useful, it is pretty much the most useful. Every so often you will run across mobs who dispel you. This means they strip away all your magic spells you have casted on yourself, and this can mean a quick death when all your defensive spells go down. Mana shield prevents this from happening. You can be dispelled, but as long as you have mana left your spells will stay up. The second effect of mana shield is to convert physical damage to mana damage. This means you can't keep it up for long, especially when you aren't a caster without being ran down to 0 mana. When you hit 0 mana the shield falls, and you are open for dispelling again. For this reason it is only useful if you are facing a dispelling mob, and not outside of those situations. The mana cost is just too high otherwise.
Honorable Defensive Mentions
Spells aren't the only thing you can do to ensure you take less damage in combat. Learning the thief spell dodge will also reduce the amount of damage you take. If you have a high dexterity it will be even more effective. On the flip side the warrior skill parry will do the same in reducing damage taken. The thief skill sneak will also ensure you aren't taking the first hit when walking into a room with an aggressive mob. This is most useful for minion classes, but for melee it can be the difference between you getting the first hit or being first hit. All of these get better as the skill increases, and they are also cheap to improve with combat points if you have some to spare.
Conclusion
There are a lot of different options to keep yourself alive. The key is to use as many as possible to build layers of protection into your character. You want the armor class to reduce damage taken, and sanctuary to severely reduce the amount of hp lost when a hit does land. Depending on the type of attacks the mob uses, the elemental shields can furthermore reduce the damage they can do to you. Then with dodge, parry, and some dexterity, you should be about as good as possible for any class. The less damage you take means less time healing and regenerating after each fight.