Multiplayer's Defense Pact...

I don't think there's any problem at all. In fact, there are plenty of veterans who can completely solo the 'Death Zone Operation' multiplayer, even defeating the hidden boss alone. Solo play might result in some missed shots due to suboptimal team composition, and it can be quite tough, especially in Death Zone-class missions, barely making it to the boss. However, in multiplayer, even if you miss a few shots, if you have allies who've considered a perfect setup, it becomes negligible. In multiplayer, what you should worry about isn't skill level but your connection. Although you can reconnect within 2 minutes if you leave mid-mission, there are rare cases where you might be flagged for an unfair departure even with an early return. Leaving mid-mission can be a burden as it prevents you from covering for your allies' missed shots, but since the co-op itself is part of the fun, most situations overlook this. I think it's a great opportunity to join and learn about team compositions, as you'll frequently encounter players who've achieved the 999 agreement.