Learn Finnish: Sinun puolustuksesi on liian heikko.

The basic noun is puolustus (defense). Nouns ending in -us that are built from verbs usually change to an -ukse- stem before any endings are added. To say "your defense", we attach the possessive suffix -si to that stem, giving us puolustuksesi.

Could I drop Sinun and just say Puolustuksesi on liian heikko?

Yes! Because the possessive suffix -si already shows that the defense is "yours", the pronoun sinun is grammatically optional. Including it, as this sentence does, just adds a bit of emphasis.

Since we are learning postpositions like vastaan (against), how would I say "your defense against it"?

You would use the postposition vastaan, which requires the word before it to be in the partitive case. To say "your defense against the attack", you would say puolustuksesi hyökkäystä vastaan.

Does the adverb liian (too) ever change form to match the adjective?

No, liian (too / excessively) is an uninflected word. It never changes form, no matter what adjective or noun follows it. It simply sits right before the adjective, just like in English.

If "your defense" was the object of an action, like "I break your defense", would change?