Use a pulldown machine that allows you to work unilaterally (one arm at a time) like the Hammer Strength isolateral machine. Add a band to increase tension in the contracted position.

Increase time under tension by keeping one arm fully contracted while the other arm cranks out reps. Here's how to do a 5-4-3 rep scheme this way:

1. Bring both arms into the contracted position, then while holding one arm there and keeping it contracted, do five reps on the other side.
2. Alternate sides so that when one arm is done with the reps, the other side begins.
3. Go from five reps on each side, then four, then three.

So while each arm gets a total of twelve reps, the "resting arm" never really rests.