My Marcus is at Level 16. All other non-DLC characters are at Level 14~18. Writing this, I think this comment has become a giant wall of text but oh well.
Unfortunately, Marcus doesn’t have a lot of options available for his Skill Card strategy; unlike Kait and JD, he doesn’t have access to Bleed Cards which are absolutely vital on highest difficulties. And his focus, which is the Lancer, isn’t a very remarkable weapon to begin with; damage-per-shot is 82 (Snub is 150), rate-of-fire is only fair, and it can only rely on Weapon Locker and Ammo Pickups for resupply. More importantly, the Lancer has trouble when Execution Rules are on and enemies who can stay on DBNO are present, such as Drones, Grenadiers, Snipers and et cetera.
He heavily relies on Living Legend (or LL) to increase his damage output to x2, but the problem is that the base duration isn’t even long enough to exploit properly.
Thus, the very first goal of every new Marcus player is getting Rifle Feedback as soon as possible, as it significantly increases the potential duration of LL. With it, you can even recover your gauge from almost-zero to full. Note that the damage you deal determines the amount of Ult recovery. You will notice the (lack of) presence of Rifle Feedback, and this is what lets Marcus briefly touch the output of other Bleed dealers and Tri-Shots.
Then, upgrading your Extended Rifle Mag is next priority. Since it expands magazine-size by 100%, Lancer gets 60 rounds, Enforcer now has 80 bullets, and Hammerburst can hold 108 shots. Marcus can reload less often and prolong his LL much more longer.
After securing those two, you may be interested in Custom Lancer and Last Ditch for more damage.
Custom Lancer provides an unconditional damage bonus for your starting Lancer, and the Lancer has acceptable damage-falloff that retains its damage-profile to some degree, so it can consistently sustain the LL duration. So I always recommend getting Custom Lancer.
Last Ditch offers an unbelievable damage-bonus that also applies to other Weapons and Heavy Weapons. It is a very tempting and good option, but it becomes more difficult to use on higher difficulties and later waves with enemy damage maximized; from a melee attack from Juvie to a missile from DR-1 Devastator, almost everything downs you with just a hit. Drones and Elite Drones (Hammerbursts and Claws) down you before you can even react too. So I stopped using Last Ditch eventually, but that never means it is a bad Card.
This is my personal preference, but I enjoy using the Brutal Efficiency as it is helpful in early waves where your defenses are not up yet and melee enemies such as Rejects swarm you. It also recovers considerable amount of Ult so I really like it. But since melee-combat become foolhardy in later waves, I totally understand if you do not take it; Damage Feedback upgrade Lv7 (27% conversion) entirely replaces Brutal Efficiency in this regard.
Usually most Marcus players pick Rifle Feedback, Extended Rifle Mag, Custom Lancer, Last Ditch so there is an empty slot for your liking.
Unfortunately Band of Brothers is bugged and only provide Stim whenever it feels like it. I still take it to promote teamwork, but its inconsistency is not worth the slot for the moment. This is why Defensive Stim is also useless right now.
So you can either take Dug In for some survivability, Lancer Stock for more consistent damage-output, or Friendlier Fire if you want to get more flexibility when trying to cooperate with strangers. I take Friendlier Fire since I view Marcus as an offensive support, rather than a full-blown dealer.
While in-game, you will want to upgrade Damage Feedback and Assault Rifle to Level 7 since Marcus’s entire tactic revolves around LL and rifles (mostly Lancer). I don’t really recommend going beyond Level 8 since the bonuses are tremendously diminished, only giving +1% for each Level.
In wave 1~10, I don’t use Lancer ammo at all, Instead I ditch the Gnasher and take Enforcer or Hammerburst so I can use those for LL, and heavily exploit the mighty Chainsaw when cooldown is on. This is because Kait and JD are the best candidates for getting the two Ammo Pickups; their Gnasher and Lancer GL are uncommon and thus can’t rely on the enemy loot for restocking. Marcus’s Lancer suffers the same problem but it does have the infinite Chainsaw that can also outright remove annoying Rejects. Though, you will have to use wallbouncing to avoid melee-attacks on higher difficulties. Once your team gets enough Weapon Lockers, consider getting a second Lancer, stow it, and rotate them after every Wave, Then you can start using your Lancer bullets extensively.
As the waves go further and enemies start doing more damage, you will want to gain Ult charges quickly by doing damage with Heavy Weapons (especially the Tri-Shot), and return to Lancer when LL is ready so you can maintain LL for you and your teammates.
A competitive team will have Del, Jack, Kait and JD. If you become their 5th player, try to find a large cover that can fit 3 people, and also provides an unobstructed view of the field. Consider sticking with Del, as Kait and JD don’t benefit a lot from headshots and will try to exploit Bleed with shotguns and Lancer GLs instead. But Del often uses Tri-Shots when repairs aren’t too demanding.