君の実力なら楽勝だとは思うが、それに驕らず、まずはクラスのみんなと切磋琢磨していって欲しい。

、、。

I'm sure you'll win hands down with your ability, but don't let it go to your head — first I want you to grind it out alongside your classmates.

Literal

Your [genitive-の] ability [if-なら] easy-win is [quotative-とは] think [but-が], that [to-に] not-becoming-arrogant, first [topic-は] class [genitive-の] everyone [with-と] mutually-refine-going [want to-て欲しい].

A teacher's or coach's measured encouragement. Three notable pieces: (1) ~なら is the conditional of supposition, 'if (we're talking about) X' — takes a topic-like role and introduces a scenario. (2) 驕らず (おごらず) is the classical negative connective form of 驕る ('to become arrogant, conceited') — ~ず is the archaic negative corresponding to ~ないで, still productive in formal writing. (3) 切磋琢磨 (せっさたくま) is a 四字熟語 literally 'cutting, polishing, grinding, buffing' — from classical Chinese metaphors for working stone — meaning 'to refine oneself through diligent mutual effort,' especially with peers. The whole phrase ~と切磋琢磨する captures the image of classmates pushing each other higher together.