Dragon Ball Z, Vol. 16 by Akira Toriyama

Cell has fused with android #17, becoming practically invincible - but not for long! Training in the Room of Spirit and Time, where a year passes for every day outside, Vegeta and Trunks have gone beyond the Super Saiyan, reaching a level of power even greater than second-stage Cell! Now Cell is the one who is outmatched - but Vegeta hates a boring fight. Succumbing to his pride, Vegeta agrees to help Cell fuse with android #18, the last element he needs to become "perfect," so he can have the honour of defeating the strongest being in the world!