Honestly, this post may seem like quite a bit of common sense, but you would be surprised how many people don't prepare for Holy Wars properly. This post will be short, it will also be blunt.
1) Stop buying damn cards for your deck and save your damn HP. You think that adding that 5k damage/defense to your deck is more important than having HP to spend during HW? If your deck isn't up to par yet, tough luck, attack weaker opponents. One card probe peoples defense before attacking. What you don't do is leave yourself with thirty damn HP right before HW you anchor.
*If you are a new player and 30HP is a lot for you, it's understandable*
2) Liquidate. Do you think getting HP for your cards is hard now? Who do you think is buying during HW? Not people paying a premium I assure you of that. If you can't get HP for them, get CW for them and trade CW for HP.
3) Know how Holy Wars work. With all the damn information out there, there is no reason for you not to know how Holy Wars work. I don't care if you're new, you should still know when it is and isn't alright to start a battle and not to attack players before the wall is down because you lower their point value. You sure as hell shouldn't start a battle if your defense leader has no points from the last war. (Happened 3x to my order in marvel war of heroes when some asshat would declare with our defense lead at 30 points)
4) Have your orders full. You don't get bonus points for having open slots.
This has been a short post on prepping for Holy Wars. Pay particular attention to point number one. Good luck and have fun!