Gloomhaven

There are 7 side scenarios that you need to complete in a single campaign in order to unlock this achievement.

4 character specific quests - one for each of the new classes, unlocked by city events. The best option to get those is to create a party containing all of the DLC characters as the city events that unlock those quests will not pop up until you have them in your party.
Scenario names:
- Unfriendly Message (Red Guard)
- Best of the Best (Hatchet)
- Warding the Void (Voidwarden)
- The Greatest Job in the World (Demolitionist)

1 side scenario from following the main quest line
Dependent on one of the choices you make during campaign this will either be The Heist or Den of Thieves, no matter which one you unlock you only need one of them for the achievement
- The Heist/Den of Thieves - depending on the choice you make during campaign. At one point you need to do either Defiled Sewers or Beguliling Sewers. You cannot do both on the same playgrought and each one of those is unlocking different side scenario. Fortunately you only need one of them for the achievement if you do character and city event side scenarios as part of the same playthrough.

2 scenarios from City Events
There are a total of 20 city events with this DLC and 2 of them unlock side scenarios. Since the single playthrough of JotL is less than 20 levels there's a chance that you won't unlock both of them but there's a workaround for this issue. Just start any JotL scenario, resolve the city event, start the scenario and immediately abandon it. When you repeat it again new city event will be resolved. You can repeat this process until both side scenarios are unlocked. The good news is while resolving city events for those scenarios there's no way of choosing a "wrong" option - no matter what you choose, the scenario will unlock. This "workaround" will also work for unlocking character specific side scenarios - just make sure you have all of the new classes in your party.
Scenario names:
- Misplaced Goods
- Agents of Chaos