Knowing this first (τουτο πρωτον γινωσκοντες). Present active participle of γινωσκω. See 2. Peter 1:20 for this identical phrase. Nominative absolute here where accusative γινωσκοντας would be regular. Peter now takes up the παρουσια (2. Peter 1:16) after having discussed the δυναμις of Christ.

In the last days (επ' εσχατων των ημερων). "Upon the last of the days." Jude 1:18 has it επ' εσχατου χρονου (upon the last time). In 1. Peter 1:5 it is εν καιρω εσχατω (in the last time), while 1. Peter 1:20 has επ' εσχατου των χρονων (upon the last of the times). John has usually τη εσχατη ημερα (on the last day, 2. Peter 6:39). Here εσχατων is a predicate adjective like συμμυς μονς (the top of the mountain).

Mockers with mockery (εμπαιγμονη εμπαικτα). Note Peter's play on words again, both from εμπαιζω (Matthew 2:16), to trifle with, and neither found elsewhere save εμπαικτης in Judges 1:18; Isaiah 3:4 (playing like children).