Visual effects such including tweening should be handled on the client not the server but

But what if someone joins while the effects should be on progress so they did not get the :FireAllClients() but actions are still being performed (character animations, damage, etc) that would indicate an action, but the visual effects are missing? How would I solve this issue?

You’re trying to make it client-sided it sounds like.

Try setting off the tween with a localscript maybe?
Make it so the tweened object is using a server script to damage the player

sometimes you let the server handle i t