Good news: you can use cheesy savescumming methods to ensure all your attacks hit and to avoid most enemy attacks.
Bad news: This is still obscenely hard and time consuming, and it's very easy to completely destroy your run at any given time if you're not paying very close attention and making all the right decisions.
- Use and breed only hunter and alchemist classes, caberjacks are absolutely useless. Vanguards of three hunters and two alchemists are the way to go. Unless you want to use the method that Schaafy describes in the other solution for this achievement.
- Build 3-4 keeps and keep 2-3 bloodlines alive since you absolutely MUST have relics passed down throughout the game to even stand a chance later on.
- Allow your vanguard heroes to grow old so they leave behind relics when they pass away, then keep passing those relics on to young heroes.
- Do NOT use Cadence weapons! These do not turn into relics when the user dies, so while they may be useful at first, you'll sorely regret it down the line.
- Pay attention to personality traits when choosing new heroes for your vanguard. Try to avoid Revelers if possible (especially if appointing a Standard!), and just try to keep the negative traits to a minimum in general. This also goes for appointing regents.
- Research Veil Armor as early as possible, and abuse the hell out of it to safely scout out areas with your hunters.
- As for items, use whatever you like. I prefer to stock up on health items, researching health vials and ultralixirs as early as possible.
- Speaking of ultralixirs, the Ultralixir Arrow (hunter) and Throw Item (alchemist) skills are incredibly handy because it allows one health item to heal several people at once.
- In keep/crucible/sagewright defense missions, it's nice to save the people appointed there, but do not take unnecessary risks to do so. If you spawn on the opposite end of the map from the sagewrights for example, you may as well just move slow and let them die. They are far more easily replaced than your vanguard.
- When building, don't go for the outer regions. Most of the bonuses (with the expection of the reduced research time one) are not worth it and will put your stuff at huge risk later on. Not to mention you'd have to do more awful building defense missions.
- Something very important to always remember is the stun effect from the shockwave when a lapse dies. The stun may only last one turn, but it can really screw you over if you forget about it. This is especially important during the final battle, as your whole vanguard is usually pretty grouped up. Be careful!
Here's how the save abusing works:
In battle, the game saves at two times; when your use up an action point, and at the end of the enemy turn. It does not save between every enemy attack. It does not save the results of the last action you made on your turn until the end of the enemy turn, so if you load the save during the enemy turn, you will be put back before your last decision. You can use this to redo an enemy turn as many times as you like, which is absolutely vital to make sure ranged enemies miss as much as possible. Especially Advanced Bulwarks.
Now here's the problem. When you make a move that's not the last move of your turn, the game does not let you pause and load until the action is complete, which means whatever you do is irrevocable. Unless you VERY quickly hit the guide button, then start, then quit game. Start the game up again, and if you were quick enough, you'll be back before your last move. As horribly tedious as this is, you pretty much have no choice but to use this method.
Everything is fairly manageable until year 151, but that is where shit really hits the fan with the introduction of advanced enemies. Those Advanced Seeds you were mowing down just fine on normal? Yeah, they now have 39HP, hit like trucks and are awkwardly hard to target. Just one of many reasons to have a few alchemists with you at all times, since their accuracy is not dependent on the enemy type.
Lastly, we have the final battle. Your whole vanguard should be at least level 7, all have relic weapons of at least level 6 or 7, and good genetic traits. If you don't have at least the first two of those, you won't stand a chance. The enemy numbers are absolutely insane, and you will have to abuse saves a LOT to ensure critical hits. If you have Advanced Hunter Armor, equip it instead of Veil Armor since stealth is useless here. It's all about offense. Stay near the chalice and destroy everything from afar with the hunters, making sure every shot is a kill. The alchemists need to kill several enemies with one flask every turn, and recharge their flasks at the chalice whenever possible. Use Ultralixir Arrow/Throw Item to heal your whole vanguard at once if necessary, and protect the chalice at all costs. Enemy attacks can glance and miss the chalice just as they can you, so use that to your advantage. Seeds and wrinklers are top priority kills, then lapses, then cradles, then bulwarks and twitchers. This battle took me close to three hours, and I only had about 40/500 chalice health left at the end. And that was with a level 8-10 vanguard all with relics around level 8-9. If you've gotten this far in Brütal Iron, you likely have the tactical wits necessary, but just know what you're getting yourself into.
Good luck, and feel free to leave any questions or added advice in the comments.