SHORT ANSWER
Start with Marcus's Gunfire / extra-shot engine route
Marcus is the clearest route for learning the pickup loop: one weapon, one firing event, and an obvious reload problem.
How to use this result. Start with the route that matches this difficulty's risk profile, then adjust for the drops and failure point in your current run.
What changes at Common (Normal)
The first measured clear tier. Steam calls the achievement Normal while the in-game ladder commonly labels this tier Common; the old /normal URL is kept for search continuity.
Create a repeatable shot event.
Vintage Musket, Geminate Shot+, and Reload Speed or Fire Rate.
Check: If the gun is still waiting on long reloads, do not chase crit yet.
Turn uptime into more projectiles.
Quick Fire, Triple Shot, Bullet Attack, then a matching element relic.
Check: The route is working when ordinary fire produces several useful events per magazine.
Convert volume into a stable boss finish.
Critical Chance, Critical Damage, Emperor's Holy Sword, and reload support.
Check: Keep one reliable damage channel; do not add a second engine just because it is rare.
Boss and crowd priorities
Use reload support and crit payoff once the target stays alive long enough for repeated first-shot windows. Keep firing discipline; a burst that spends half the fight reloading is not a boss plan.
Extra shots, fire rate, and a weapon that spreads pressure are the priority. If packs are the failure point, Bullet Attack and volley effects beat a greedy single-target card.
When the recommended core is missing
Keep the physical element, choose Fire Rate and Reload Speed, and use Demonhunter Crossbow or any reliable physical family.
Do not: Do not hold empty slots for one exact skill while the current weapon has no uptime support.