楼主的中文有歧义,所以可以译为两种情况:
1、This is the last time; I swear on my soul.
这是最后一次(表示以后不再做某事),我以我的灵魂发誓。
2、For the last time, I swear on my soul.
最后一次,我以我的灵魂发誓。(表示这是我最后一次干”以灵魂发誓“这件事)
For the last time, I guarantee to my soul.
但通常是我以我的灵魂保证,
I swear on my soul.
For the last time, I guarantee/assure to my soul.
For the last time,I swear to my soul.
It's the last time. I promise to my soal.