Hello, I don't have much experience when it comes to backend services ( or in general)

Basically, I'm making a semi-online game. It's an action game where the player fight monsters. But I want to add a "Raid" system where multiple players in a certain group will fight the same enemy and eventually share the loot when he dies.

The enemy will have a health bar, (let's say 1000 points) that will be shared among that group of players, so when a player deal some damage (200) the other players should immediately see that the bar is now at 800 points.

 the players don't play together in realtime, meaning that they don't see or interact with each other. the only connection between them is that enemy's health bar, which if a player is just sitting there doing nothing and a second player is dealing damage, the first player should see that bar getting lower based on the second player's actions.

Two questions:

1- What is the most efficient way of achieving this in real time, or even every few seconds.

2- How to deal with offline players? does Gamesparks provide some sort of "auto-synch" like Firebase?

Thank You.

Thanks for your answer @Vinnie
This does seem like what I want to achieve, but how will it handle multiple requests from many players in short amounts of time? meaning that whenever a player takes an attack action I want that to be synchronized with the other players, this could happen extremely fast and from a large number of player (think of a clan of 100 players or even more)

I also would like an answer to my second question about how to deal when the player goes offline, is there some sort of caching and auto-synch feature that happens when the player goes back online?

Question about Real-Time multiplayer,  That "Raid" system I'm implementing might go on for hours or even days, depending on the number of players and their actions, would it still be efficient to use RT multiplayer and handling requests from large numbers of player over that period?
I don't want the "sync" process to be instant, it's okay if it takes couple of seconds.

I'm sorry I've been asking a lot of questions and sounding like a complete noob, but please bear with me.
Let's say I'm using a RT server:

1- This is just a server that is available whenever a user connects to it? Is it

2- Does it have the same limit as in the link above (Fair usage policy), or is it separate from that?
3- Once the user connects to that server, it will automatically sync the client with the most recent data, and whatever actions the player make will have an instantaneous effect on all the other clients?.

Finally, in general, what counts as a single "API Request" for example in a messaging app, does every message sent or received considered an "API Request"?

Hope I'm not much of a burden :)

A new RT session will be created on the server when a match is made and at least one player connects to that match. It is not quite the same as a dedicated server.

1- When a player starts a new session, can other players join him in the same session without him directly inviting those other players?( It's a clan based game after all) 

2- Is there a way to save and retrieve data from previous sessions? for example, two players in a session collectively did 300 damage to the enemy which had 1000 health points before. is it easy for when another player starts a new session to see that the enemy now is at 700 health points?