I wonder if Serie plans to die to get these mages to exceed her expectations and create the future she can't. Maybe that is why Serie tried to get Frieren to leave due to how strong she is. We have already seen how skilled the assassins are, so it is unlikely the Serie doesn't know that they are at some risk. She mentions achieving a future that she cannot. Indicating that the future she wants is something she will never achieve, even with the help of the other mages, so she might believe that her death would push the other mages to achieve that future. Serie also mentions the mages she brought being able to exceed her expectations, indicating that she believes that the current way she does things in lacking, and that she understands that her exceptations of criteria is not allowing for the future of hers to be created. This may be also why she doesn't tell them the plan because they would all surely disagree. Sure Frieren and Serie have beef, but I don't see the all other mages disagreeing purely out of spite. Although this is just my gut feeling from the first read of this chapter so I could total be off on the theory. Another entirely possible thing is that Serie doesn't plan to sacrifice herself but instead be bait, forcing the mages to step up at the final moment to prevent her death, therefor breaking through her expectations. I am unsure of her intentions as to whther she wants all the mages she brought with the exceed, creating a new group of peak mages, or to just create another peak mage like Serie, possibly through the deaths of their comrades.