Did that scenario at least 5 times solo and twice recently with 6 players. It was hard with Lini solo, but besides that, not at all.
It's a very combat heavy scenario so if your characters aren't good at combat, it's a problem.
1) Everyone should stack on the 1st 1-2 locations (depends on group size) until they are closed (with the exception of Meresiel). Minimizes the chance of the boss escaping.
2) Try closing Goblin Fortress first, when everyone is fresh and at full health. If you get Gogmurt on this location it can be tough (+2 to goblins with another +1d4 from the scenario), but at least with allies on that location they can handle the second combat check. Or heal you if you lose a lot of cards.
3) Play with a healer (with at least 2-3 cure cards) like Kyra, Lem, or Lini. At least if you get in trouble, there's a way to recover.
1) Put upgrades into combat stats, at least initially.
2) Sajan is weak initially without blessing upgrades. Either harvest blessings from scenarios with lots of blessings, or consider using a different character.
3) Mersiel. Should always solo a location. Don't be afraid to recharge stuff in your hand you don't need, even when you don't need to buff your combat check. Don't be afraid to discard when you're doing an important combat check. Don't forget you can evade any encounter.
4) Lem: Not that good in combat initially, maybe have him focus more on cures. Or using S&S Lem.
5) Seelah: Cannot afford to lose her Cure when recharging. Use allies and/or her power to avoid this happening. Don't be afraid to use her power in almost every combat or check. Don't be afraid to use her scouting power on monster heavy locations (Nettlemaze). Should have all magic chainmail, so she can recharge her armor to get other cards. Should have all 2H weapons, they are just more effective.