Gunfire Reborn

This achievement requires you to defeat all the monsters you encounter in the game, including all the bosses in the three main worlds as well as Pole Monarch in the optional fourth world. Very likely you'll unlock this achievement after beating Monarch following a lot of failed playthroughs whilst beating the initial and alternate bosses of the three worlds when you've mastered the former.

To beat Monarch, you'll need, and I mean REALLY need to acquire a loadout in terms of weapons, weapon inscription, Ascensions, and scrolls that will give you a high damage (or damage per second output) rate. Another thing to consider as well is the choice of hero and their exclusive set of skills. For the character I chose that got me to beat Monarch, I went with Ao Bai and his Dual-Wield skill, allowing me a good chunk of time to unload with both of my weapons (A pair with normal and large or special ammo is a suggested optimal combo, but utilizing pairs that both use normal ammo works too, and may actually be the better option if you also get the Ammo Expert Ascension [Which despite the description that you recover x ammo per second while using Dual-Wield, it's actually active at all times!]) Also make note of the Gemini Inscriptions on your weapons, they'll activate if the descriptions match on both of your weapons and make you more powerful. Prices for Gemini Inscriptions on non-branded weapons range from 0 gold at base level to 1000 on +5 level (I think you get better sets if you hold off on paying until upgrading your weapon to a higher level, not sure, but was taking notes after watching some YouTube clips of players on how they beat Pole Monarch).

At the start of battle with Pole Monarch, he'll just stand and wait for you to make your move before he goes in with a sweeping set of sword-slashing strikes. If you have a high DPS rate you won't face much else from him, but if not he'll put in some combination attacks that make avoiding damage a lot more difficult later in battle. As you blast away his health there'll come two instances he'll call for a giant syringe device to give him temporary invincibility as well as inject elemental abilities. Destroy the syringes in these circumstances. Then keep driving down the health bar until it's about 95% empty, and he'll then dig his sword down into the floor in a last ditch effort to fully heal and go full Mach 2 in regard to attack sets. You have 10 seconds to destroy the buff of armor HP as well as his speck of health (Hence why you need your DPS to be as high as possible in the leadup to the battle.)

As for weapons, I used Porcupine as well as Crimson Firescale, both at level +13 and sharing the Gemini ability of planting a spore that explodes when spores from both weapons meet each other. The higher the level of at least one of your weapons, the higher the damage output. But remember, weapon damage is just one of MANY components that add to your total damage rate. Ascensions and scrolls that can boost your damage rate are just as important. Prioritize them when you have to pick one when multiple are offered after beating Vault challenges as well as bosses. 😎