Complex sentence: The attacker can then compute "ENC k 1 (P)" for all possible keys "k 1" and then decrypt the ciphertext by computing "DEC k 2 (C)" for each "k 2".
Split into simpler sentences: The attacker can then compute "ENC k 1 (P)" for all possible keys "k 1". Afterwards s/he can decrypt the ciphertext by computing "DEC k 2 (C)" for each "k 2".