Really depends on the universe, but we can theorise that time travel defences work on the same principles as time travel, so you probably have to be theoretically capable of it to actively defend against it (but universes with splitting timelines have pretty good inbuilt defences).
First off, relativistic is not FTL. But for either case, it depends on the speed of your sensors, FTL sensors might allow you to avoid attacks, and sufficiently powerful force screens could absorb impact.FTL Travel
(FTL attacks, attack while FTL, FTL defenses, FTL senses)
If your foe is moving and attacking at relativistic speeds, can you win if you are not?
But as any ship moving relativistic relative to you experiences you moving at a relativistic velocity the increased speeds mainly makes engagements much shorter with more energyn being delivered. Assuming ships aren't accelerating, then we might have to factor in time dilation.
For FTL stuff it really depends on the system being used, it can be anywhere from totally meaningless to requiring us to deal with infinite or negative energy.
Short answer: no. Long answer: while you were preparing to fight back the enemy took apart your planet for precious materials.WMD
(Divinations, defenses, size does matter)
If your foe is destroying things at the planetary scale, can you win if you are not? How many worlds does each side have, and how fast can they destroy them?
Well, kind of. The Death Star is impractical and beatable, exterminatus probably isn't in the long term. The real problem is closing of the possibly quite broad technological gap, and destroying things is anywhere from glassing them, to taking them apart for resources, to just blowing them up.
It basically comes down to four things: the numbers a universe can field, the power of the weapons they can practically field, the strength of the armour they can put on things, and their ability to deal with outliers on the opposing side.Personally, I hope that the final answer isn't so boring as to obviously fall along such lines (plus things like universe size vs movement rates or range of senses / attack range).
The first qualifier might as well be 'can they control orbitals', to get rid of rods from orbit being a problem. Time travel defences are a good second qualifier. Once that's done it's mainly going to come down to who can keep their man on the objective the longest.