(Black to play, and only move is to block with rook).
This is reached from your position after:
1. Ra6+, Kf5 2. Rb5+, Ke4 3. Ra4+, Kd3 4. Rb3+
You should be able to force a trade of rooks by either making it his only option, like above, or creating a situation where he will otherwise be mated in a couple of moves. You should move your rooks one row at a time to create.