[@Gunther] Sotamies would be correct, as jääkäri is a branch specific rank. The rank jääkäri was adopted from the German Jäger, as Finnish volunteers were given soldier training in Germany during the First World War. And sissi is a term for a soldier, who uses guerrilla-like tactics, like ambushes, infiltration and such things. There's also a slang term "sissijäynä" (sissi prank) for booby traps.

I'll write more thoroughly, when I get home, as I have only a phone at the moment.