While “Frozen Soul” is in effect, you cannot activate “Last Turn” in the same turn because “Last Turn” includes conducting a Battle Phase.
When you activate “Frozen Soul”, and effects are chained that result in the Life Point difference being less than 2000, “Frozen Soul”’s effect still resolves.
When you activate “Frozen Soul”, and your opponent skips his next Battle Phase somehow, his Battle Phase of the next turn is skipped with “Frozen Soul”.