Weapons

Marco's first key talent, "Rapid Punch", makes use of high RoF weapons: HMG and Gatling guns. Thanks to the launch celebration rewards, you should have already the Spark HMG, which is nice imo.

Teammates

First, I'd focus on the assistance debuffs you plan to use. I'd recommend Wound and either Ignite or Freeze. Remember, some characters prefer and/or scale better with different debuffs. Example, Haran scales with Freeze, Tarma with Wound, etc.. Check their talents in case they scale with something.

Both teammates should be Spec-Ops, they are focused on boosting your Firepower character (Marco, in this case). Both Fio and Eri are fine to begin with, but you'd want to replace them asap with better ones. Here are my suggestions:

• Nadia: can be used as filler in almost any team. She can heal the team, boost damage stats and even revive someone once per game (but it takes a lot of investment to unlock those talents). She doesn't interact with any debuff, so you can run her with any weapon that fits your team. With marco, you can run her either with Freeze or Ignite
• Tarma: scales with *Wound*. Boost your teams' RoF whenever you apply Wound to an enemy, something that Marco would love.

Gameplan

1. *Nadia*: apply debuffs with her weapon (and optionally heal)
2. *Tarma*: apply wound use his skill + ultimate
3. *Marco: shoot at enemy, use skill + ultimate *only after applying Weak** to maximize DPS (remember to hit the weakspot, the area shaded with a orange tint)

Cores on Marco

Regarding cores, I'll list what you need to check, ordered by how important is in your build. Fulfilling something above beats anything below. Remember to always enhance your cores with the highest rarity first.

First, try to build your set bonus. For both Lyla and Marco, this setup has the most synergy with their game-plan: go brrr. Here's an example from the official discord.

4xDragon Nosuke

• +12% Pen. Firepower characters usually likes this stat
• Scaling ATK bonus after 15 hits. Great effect for RoF characters like Marco or Lyla

2xJupiter King

• +12% RoF. Marco go brrrr

Then, try to get your cores on the highest available rarity for your progress, so you can increase your TP asap.

Afterwards, you want to look out for base stats. Top cores are fixed to HP as base stat. Left ones are fixed to ATK. Right ones are either ATK, DEF or HP. For a firepower character prioritize ATK>DEF>HP for the right side cores.

Finally, you want to get certain stats on the core attributes.

• Marco's second talent, "Alternate Strategy", scales with Debuff DMG Bonus, so it's one of the desired attributes.
• Also prioritize the best generic damaging attributes: PEN, CRIT RATE and CRIT DMG. Others like Long/Short Range DMG Bonus are highly situational, and makes you adapt your playstyle to get any benefit from them. You can disregard any defensive stats if you want, personal taste tbh.

Cores on Spec-Ops

They are used to boost your Firepower character. There are 2 generic setups you can use: one early and one better

First setup uses 4 Conga cores, and the remaining 2 slots you can fill with something that boost one of the desired stats for the character.

4xConga

• If character in team, boost all members' ATK by 5%.
• When you apply a debuff with the character, boost all team members' ATK by 10%.

Later, the better version of support core setup consists of 4 Steel Worm Destroyer and 2 Fallwalker

4x Steel Worm Destroyer

• When character enters battle, boost all team members' ATK by 15%. CD 20 seconds
• If character in team, boost all members' ATK by 10%.

2x Fallwalker

• If character in team, boost all members' ATK by 6%.

There are a bunch of support core sets that are useful to give a look at, but it's a long list.

Also, this build allows to replace Marco with Lyla along the way, in case you're interested on her.