Skul: The Hero Slayer

Note: This solution is not specific to the First Hero boss. These are general notes on save scumming, which I used to practice and defeat other bosses.

Save scumming is possible. If you quit to dashboard before dying, when you restart the game will load at the beginning of the current level. This makes it possible to practice individual bosses and keep your current run from ending badly.

Some random things are different each time the game is reloaded. Here are my findings.

Reloading the level in this way does not re-seed end-of-level rewards, or the items available in the Black Market (most of the time*), including the refreshed inventory.

Reloading the level in this way does change random outcomes of freed NPCs:
* The blessing from the Wolf Shaman
* The item from the Noble Painting
* The skull from the Sleeping Skeleton

The item duplicated by the Slime Prince is the same every time, but if you destroy that item first, something else will be duplicated; you can go down the queue destroying more items until you get an optimal result.

Random inscriptions on the Prince's Box and the Clone Stamp are random with each reload (though the items' appearance is not). Given that there are 36 different inscriptions, I don't recommend farming too heavily to get something you want.

Reloading the special room with the huge chest randomizes the contents of the smaller chests, but not the contents of the huge chest.

*Occasionally, the first set of items sold by the collector will change the first time the Black Market is reloaded. I didn't determine any specific cause.

The dev console was patched out, so you'll have to get it legit now.

I managed to pull this off, but not without taking full advantage of exploits.

NOTE: Apparently this game has been patched again and the exploits listed in this solution no longer work. For instance, enemies that spawn more enemies no longer do so forever. Without that exploit, getting this achievement with the reaper skull will be much harder. If someone comes up with a better solution, let me know and I'll direct people to go read theirs. I no longer have access to this game since it was pulled from game pass, and I can not confirm how much the game has been changed and which exploits I've pointed out have been removed.

One of those exploits is save scumming as outlined in Skeptical Mario's solution. Skul autosaves on every level. As long as you don't die, you can quit the game and restart the same level you got to. If you do die, the game will autosave immediately ending your run.

This very much comes in handy with The First Hero. This boss has 3 phases. All 3 have attacks that trigger once the health bar hits a certain %. In particular, the 1st and 2nd phases have attacks that fill the screen blowing so much crap everywhere so quickly, a no-hit win here will be elusive. Especially for the 2nd part which sometimes bugs out and he'll immediately go into full screen attacks if you hurt him fast enough.

The best way evade these attacks and get this (and any other no-hit boss achievement) is to defeat him before he can use them which, really, is totally dependent on how lucky you were on your run.

I'm sure you've looked up videos on youtube on how to get this achievement, and you watched someone fight the final boss with an awful skull for a solid minute somehow magically evading everything including the BS 'how TF am I expected to dodge this?' full screen attacks.

I basically murdered the First Hero in 10 seconds with the following:

SKULL 1: Reaper Skull

this solution is going to be tl;dr, but its all useful info.

Reaper skull is extremely rare and typically won't be found early game. The reaper skull can break the game since it has the ability to collect souls. Every soul collected gives +1% magic damage. Enemies killed by the reaper's attacks SOMETIMES give you a soul. Enemies killed by the reaper's swap attack (the attack done when pressing to switch to reaper skull) ALWAYS give 1 soul 100% of the time. Even better: some enemies will produce a 2nd soul when killed with the reaper's swap attack.

Certain enemies in the game will continuously spawn more enemies you can use to farm souls. The 4th area, for instance, has priest enemies that will summon 2 enemies at a time forever. (NOTE: this may have been changed in a recent patch. Again, I don't own the game, I played and completed it on gamepass so I cannot confirm.)

I spent about 2 hours farming nearly 2500 souls with the reaper.

Key to farming souls is avoiding and discarding items that will deny kills done by the reaper's swap attack. This means no item abilities like sprites or inscriptions like volcano or lightning.

You also need a 2nd skull that won't kill anything when you swap to it. A perfect one that has no swap attack is

SKULL 2: Mummy skull

If you have these 2 skulls, the rest of the game will be methodically and patiently killing everything with the reaper's swap attack.

for everything else:

REAPER SKILLS:
need: Fissure of the netherworld.
avoid: guillotine

the other 2 reaper skills are useful. I had harvest which deletes about 5% of the boss' life bar when used, but is far more useful for freezing the boss in place while he's taking damage inside fissure of the netherworld or my quintessence.

The other skill is useful as an extended and longer lasting forward dodge.

I wouldn't have managed the no-hit run without the following:

QUINTESSENCE: Archdemon. I got lucky finding this. At +2500% magic damage, 1 use will quickly end any of the First Hero's phases.

ITEMS: I won't provide a list of items, offering instead the following advice: whenever you get to a store, spend all of your money rerolling the items the collector has for sale just to see what he has. Quit the game, and reload. @50% of the time, the starting 3 items will be different upon reloading the game and the new line up will persist between consecutive reloads. All items available for sale after that will remain the same. This is EXTREMELY USEFUL for finding and getting the best items in each store and knowing when to stop rerolling and save your money for the next one.

- The inscription system is stupid and distracting. You want to instead focus on the unique abilities of each item which offer way better bonuses, especially if they work off each other (eg nether mana suppressor + ritual staff).

- Crit is king in this game. You want + crit % and + crit damage any chance you get. Thief's Black Steel Dagger is S+

Items from slime: the game will pick a specific item box to dupe. If the slime dupes a junk item, you can save scum and destroy items until you have the specific item you want in the box the slime dupes. If the item you want duped is in before the box that gets duped, you have to destroy everything that appears after that item. (if you're confused, the item boxes fill up left to right basically in chronological order 1-9. You want item 3 duped, but slime dupes item 5. Delete everything after item 3, and slime should dupe 3). Sometimes slime will deviate from what box he dupes after deleting items, but its very rare.

If you read this far, here is what I did to defeat the First Hero without taking a hit:

phase 1 (1st life bar): I used Fissure of the Netherworld at the right place and time and was lucky enough to kite him into it few a few seconds. At +2500% magic damage, he didn't last long. I don't know how I managed to do this without getting hit tho.

phase 2 (golden armor): This bugged out on me a little. He skipped summoning swords and went right into full screen attacks. I used the archdemon quintessence the moment I was able to damage him and I froze him in place with reaper's harvest once I noticed what he was about to do. This managed to melt his health bar before was able to do anything.

phase 3 (weakened state): This part is no where near as bad as the others. As long as you stay away from him, the first hero can't get you. giving you plenty of time to recharge an ability. The only thing to worry about is his full screen attack he'll do at 1/2 health. 1 use of fissure pretty much nuked him instantly for me.

This is how I got The God Of Battle. Only had to save scum the fight once.

I legitimately don't know how a person with decent-to-good skill is expected to get this achievement without taking advantage of the game's exploits. The timing is so tight on some of the boss' big telegraphed attacks, its impossible to respond in time unless you're Asian or a freak.