After a minute of planning, the two of them were able to come up with a rather simple plan. Part of the reason for its simplicity is because they don't have much time to make a complicated plan and another reason is that there is no need to do so.
They only had to keep 4 things in mind while making this plan:
1. Steal the High-Grade Spirit Stone Mine Source.
2. Make sure nobody suspects Mira of stealing the High-Grade Spirit Stone Mine Source.
3. Don't kill the Branch Head.
4. Don't kill the Basilisk.
As long as those 4 requirements are met, then Mira will be able to get a Spirit Stone Mine Source without having to lift a finger. Of course, they both knew that things wouldn't be so simple as they are dealing with a Mythical Beast and a high-level cultivator who has been planning this expedition for years.