1/3 Attack on PoS?

For a Proof-of-Stack system, is it feasible to, instead of performing a 51% attack, take advantage of the finalization phase, obtain 1/3 of all validators and deny blocks to slash other validators first?

Also, how does the attestation phase prevent being spoofed?

Edit: Basically, if I send you a block, how do you know this block is legit? And how is this prevented when a party controls 34% of all stakers?