How to be safe!

In Warmachine, there are three fundamental ways to keep a model (usually your caster) safe. I will go over them today. Briefly, I call them You Can't Hurt Me, You Can't Hit Me and You Can't Touch Me.

You Can't Hurt Me is simple. It means setting yourself up so that you will take little to no damage from attacks. The canonical example of this is high ARM stacking. Models that rely on this method include Terminus, Venethrax, the Devastator, and in fact much of Khador. This is, in my opinion, the weakest of the three defenses, as there are numerous ways around it-- everything from autoplinking via Sniper or Anatomical Precision to rolling four 6s on a Bane Thrall's charge. However, the tradeoff is that this is often the easiest defense to set up. You just stack armor buffs or start with an extremely armored model (Devastator, Demolisher) etc. Furthermore, every Warmachine caster has access to focus camping as a strategy, which will (in almost all cases) push one's ARM up to 20 or even past it.
Of course, it's not just ARM stacking. Effects like the Agonizer's Gnawing Pain, feats like Rasheth's, Ossyan's and Fiona's, and other effects that reduce STR play into this strategy. There's also an offensive side: Disrupting enemy warjacks or taking out the Body system on warbeasts can reduce the number of damage dice they throw at you.

The second defense, You Can't Hit Me, is equally simple. It means setting yourself up so that attacks directed at you are likely to miss. The canonical example of this is DEF stacking, such as Quicken on Ashlynn d'Elyse or the rock wall of Janissa Stonetide. Other examples include Iron Flesh'd Kayazy, both Morghouls (and eMorg's feat), Dug In models, etc. This is in many ways a stronger defense, because a model that is not hit suffers no ill effects at all. However, the downside is the existence of knockdown/stationary effects in the game, which completely obviate any precautions you have taken, and the fact that if an attack connects you are often quite frail. This defense is also not particularly hard to set up, but thinking outside the box can help: terrain is your friend here, including hills and forests.
There's an offensive side here, as well-- Disruption will prevent boosting to hit, and taking out the Mind makes beasts less accurate. Models with Powerful Charge or Cavalry get bonuses to hit when charging, so denying them the charge can help preserve your DEF-based protection. Finally, some models CMA or CRA to improve their accuracy, so the ability Perfect Balance falls into this category.

The third defense, You Can't Touch Me, is often the hardest to set up but the best of all. If no attacks can be directed your way, the chance of being hurt is reduced drastically. Your opponent's accuracy and hitting power, as well as your DEF and ARM, become irrelevant if they're not attacking you. There are lots of ways to pull this off. Many of them are feats: Saeryn, eSkarre, eHaley, and Zerkova all have feats that deny your opponent the ability to attack you. Other ways are more subtle: Rasheth and pSeverius survive by simply staying a huge distance from the frontline. Staying out of LOS or having Stealth can help, although there are models that ignore these restrictions: it's important to note who will and won't be able to attack you through your protection. For example, if I am running Rasheth in a Krea bubble behind a forest, the only models in my opponent's Retribution of Scyrah list that will be able to hit me are his Mage Hunter Strike Force, who cannot hurt Rasheth at all (since they only roll 1d6 for damage against him due to Impervious Flesh and his ARM is 16 due to the Krea, whose benefit is not ignore by Arcane Assassin as it is not a spell if she cast it herself). Thus, I am safe. If, on the other hand, my opponent is Dawnlord Vyros with Hydras, I am in danger as the models he has that can ignore my You Can't Touch Me protection are also capable of hurting me.

The above example was a bit convoluted, but I hope it demonstrated how stacking different types of defenses can help keep you safe, and the importance of noting which models are capable of striking through your defenses. You Can't Touch Me can happen in many different ways: tying up ranged attacks in melee denies them the ability to shoot, blocking charge lanes denies enemies the ability to charge, and using elemental immunities (especially Fire against Protectorate and Legion, Corrosion against Cryx and Legion, and Lightning against Cygnar) can render certain models useless. Spells like Crippling Grasp and Rebuke form the offensive side of this, preventing enemies from engaging at all.

The safest caster is the one who is not being attacked at all, but if you must be attacked, you should ensure that the attacks will either miss or do no damage. Warmachine is a game of assassination: once you have identified the threats (as I went over in a previous post), you should figure out how you are going to defend yourself from those threats. Which philosophy of defense will you follow? How will it be enacted? What are its weaknesses? Plan, and you will stay safe.