Just pop in and out of cover and blast away at close range. Real easy to get marks accidentally just pop shotting even to extreme optimal range of shotgun without aiming for head. Marks almost guaranteed when aiming for heads. When targets move out of range or as they are approaching use the MDR to gain marks and work the armor down. Other good option for shotgun is Tactical Super 90. Backup weapon to replace shotgun when not practical is an M700 Carbon with Destructive, Prepared, and Elevated with same mods as Urban MDR. Insane damage with full marks. Use pistol to build up skills if/when needed in a pinch and also for quick marks while doing so. Roll out Expert for Coolheaded as the free skill to bypass the electronics requirement.
Pulse is amazing for upping damage output, and performance mods can be switched out for crit damage or duration if preferred. Skills are real flexible here but you want something that can heal you for sure if solo, and mobile cover is also pretty good. Talents should speak for themselves.