This isn't an escort mission, it's a defense mission
Based on how this question is phrased, it does not sound like you're under a significant time pressure, as such, this seems to be less of an escort mission and realistically can be a defense mission.
The greatest strength that your team has is numbers, which gives you an extremely high chance of success in a defensive scenario. The second greatest strength you have is that your team appears to have multiple ranged combat options and the opponent's choices for ranged combat is particularly limited.
With that in mind, the biggest threat appears to be the assassin who has the ability to move into your ranks and must be ignored when they do so.
As a general rule, when I've LARPed on the side of the cast for large battle situations, we are often dealing with a significant disadvantage from a manpower perspective (think 4-5 players for every NPC). We'll play monsters that are far stronger than the players, but given we're not trying to actually hurt each other, we're still fighting with good control. As a result, of this manpower disadvantage, our primary tactics focused HEAVILY on drawing out and isolating players into 1:1 or 2:1 situations where our stronger monsters could more easily overpower them. Usually we'd then turn them into subservient undead and thus bolster our own ranks in the process.
With all of this in mind, I'm assuming that your team does not practice any sort of formation drills. However, this doesn't mean that simple tactics aren't viable for your group. As a general rule, your goal has to be one where you focus upon keeping your group together.
The general tactic I would recommend for your group is that once you encounter the enemy you engage with the following tactical positions:
- Side Flanks: Shields need to go to the sides. The primary purpose of the shields is really just to keep enemies from flanking your group.
- Front Line Your melee combatants need to occupy the space between the shields and focus just on parrying and defending themselves. They don't need to focus upon attacking unless a singular enemy is isolated in which case, have 4 people try and cut them down. (they can't block them all) If you have an excess of shields, you can put one of them square in the middle of the front line.
- Blasters All ranged attackers need to go behind the front line and should focus their fire on a single opponent. Your enemy is just as dangerous with 1 health as they are with 5 health. So take them from 5 to 0 in one volley.
Injuries: So let's talk about injuries, from what you've stated about injuries, they just don't seem to matter unless they hit someone's arm. If someone on the front line loses a leg, who cares? They don't need to move from that spot anyways. Don't bother healing it until the enemy is routed. If someone's shield arm is injured, it's highly recommended to heal that ASAP.
Movement (Forced and Otherwise) When it comes to these tactics, my general advice is don't move. However, if forced movement causes someone in your line to move, guess what, you all move. The reason for this mindset is that you don't want anyone to be isolated. So if a member of your front line is hit with a forced movement effect to 30' behind everyone, then EVERYONE should retreat with them.
These tactics are generally very basic and can be implemented with 5 minutes of discussion with your group and generally one person focusing on keeping an eye on things (for example, directing focused fire at an isolated enemy or identifying when a shield is injured).
With regards to the enemy's Assassin. Since they lack the ability to insta-kill anyone, I'd recommend simply keeping a tight formation and unloading once they deliver their knockout since it will make them a valid target and if in a tight formation will result in a lot of hits in rapid succession.
