A topic from AEG's L5R CCG Rules forum, copied for Oracle of the Void from a copy of the forum saved in October 2015, so pages it didn't save are missing. Players' names are left out; the rules officials are named. Rulings are as they stood on these dates; later rulings, the rulebooks and the Rules Archive may differ.
Unstoppable Force: Battle: Target an opposed unit: Give each Personality and Follower in it +1F. Before the next time this turn another player's effect destroys the unit or a card in it, negate that destruction.
Since it doesn't change the state of the card as a cost, can i play it targetting an opponent's unit? It will negate the destruction from my cards or from theirs?
can i play [Unstoppable Force] targetting an opponent's unit?
Yes, assuming you control any units at the current battlefield and you target an enemy unit there (see the rules of Presence and Location).
It will negate the destruction from my cards or from theirs?
From your opponents' effects, because "another player" is related to who plays Unstoppable Force, not to who controls the targeted unit. For example if all players pass, the targeted unit may be chosen for the delayed effect of Mountains of the Phoenix and it will not be destroyed if this Terrain was played by any of your opponents.
Note that your Unstoppable Force will not prevent your opponent from destroying the unit (or a card in it) as a cost. For example he can still destroy its Personality to pay the cost of Desperate Battle (a Strategy from Death at Koten) if this Personality is unbowed and the only one defending Personality whom your opponent controls at the current battlefield.