In plain words
Thus knowing that which is beyond buddhi, steady yourself through yourself and conquer, mighty-armed one, the difficult enemy in the form of desire.
Word for word
Thus having known that which is beyond buddhi, having steadied oneself by oneself, slay, O mighty-armed one, the enemy in the form of desire, hard to get at.
The commentators
Bhagavadgītābhāṣyam
ततः किम् —
एवं बुद्धेः परम् आत्मानं बुद्ध्वा ज्ञात्वा संस्तभ्य सम्यक् स्तम्भनं कृत्वा आत्मानं स्वेनैव आत्मना संस्कृतेन मनसा सम्यक् समाधायेत्यर्थः । जहि एनं शत्रुं हे महाबाहो कामरूपं दुरासदं दुःखेन आसदः आसादनं प्राप्तिः यस्य तं दुरासदं दुर्विज्ञेयानेकविशेषमिति ॥ ४३ ॥
In plain words
And from that, what follows? —
So: once you have recognized the Self that stands beyond the intellect, and steadied yourself, holding yourself firm by your own purified mind, fully composed — that is what is meant. 'Slay this enemy, mighty-armed one, this enemy shaped like desire, so hard to overcome' — hard to overcome because it can only be approached with real difficulty, and it takes many hard-to-fathom forms. That is the sense of it.
Close to the Sanskrit
From that, what? —
Thus, having recognized — known — the Self that is higher than the intellect, having steadied — made firm — [the mind], having composed oneself fully by one's own refined mind: this is the meaning. 'Slay this enemy, O mighty-armed one, which has the form of desire, hard to overcome' — he whose approach, attainment, is [possible] only with difficulty, him [call] 'hard to overcome' — having many distinctions that are hard to fathom. This is the meaning.