Massing at Osgiliath is a special scenario, and not exactly because it is POD. It has a special feel to it. What players that revel in both solo and coop play shall experience is, that unlike many other quests (Dol Guldur or Return to Mirkwood for instance), Osgiliath scales much in favour of the solo play. Not only it is easier to keep the threat level of one player low enough in the beginning (and thus omit the ominous Wolves), with each other player the important Rangers of Ithilien ecounter cards will prove less of an assistance.
Just as scaling is often an issue, the importance of the first round is again a great factor in this quest, just as luck of an opening draw (and first round revelation). Some heroes and cards can solve the opening trouble. Such as Boromir who is best for dealing with multiple enemies. And Blade Mastery is the one card players should seek to have in their opening hand when playing Boromir. To give 2 player game as an example, with Boromir in the line-up it is likely the two Wolves of Mordor shall engage on round 1. Make sure that both engage the Boromir player, play Blade Mastery on Boromir and see him defend and destroy both, taking 2 dagame if you're lucky not having a nasty shadow effect.
Dúnhere is another combatant who can be in his prime at Osgiliath where many enemies engage at high threat values. And making Dúnhere stronger is constantly getting easier with new cards broadening the players' pool.
Osgiliath is not one-sided into fighting by any means. Beside the Witch-king most enemies are neither very strong nor too sturdy. It is the questing odds that often need to be overcome. The initial threat is 4 times the number of players and "doomed" and "surge" effects swarm the encounter cards; and cancelling "when revealed" will be of little help against either. It is not to say that A Test of Will will be of no use, but more often than not, it shall not be a definite answer to treachery cards.
The following fact is what makes Osgiliath most special among the current scenarios. It is that the encounter and shadow effects vary considerably in their potency throughout the game. In some quests, there are few deadly cards to always fear. Here a card which is most dreadful at the start can be most welcome later. Just to give a few examples, Dark Pursuit is often devastating on round 1 but more than pleasant in the end-game. Wolves of Mordor and Captured Watch-tower's shadow effects are easy when taking undefended attack from Snaga Scouts but can ruin the game when defending against the Witch-king. The poisonous location is no matter during the staging but can kill a number of Rohan forces after being revealed as a shadow effect (West Road Traveller is a proper answer to this one - but it is hard to have her handy especially with Cut Off! about).
As for a few prominent strategies, scouting with Denethor and Henamarth Riversong may be very useful (especially in avoiding disastrous shadow effects and anticipating varied outcomes of the staging in order not to exhaust too many characters for questing, or not to become too rushed with progress before the Rangers come to aid the party). And so shall the Dwarven synergies boosting both willpower and attack.
Further to the benefit of the design of Massing at Osgiliath, the finale is often exciting, especially in solo play where it is not easy to quest hard enough and at the same time fend off the Witch-king repeatedly. With more players, the road to stage four shall be more bumpy but the retreat to Minas Tirith less risky.