Target Practice challenge requires you to shoot down with vinegar at least one ranker on each ladder.
Vinegar is used to shoot down rankers that are stuck (AFK or walling) at top-1 position of the current ladder OR to protect yourself from those shots.
The promotion should be unlocked in order for 'Throw All Vinegar' button to be available.
Every second the ranker spends at top-1 position makes them lose 0.25% of their current vinegar_count. This means that their vinegar_count will deplete from the initial value to 50% in 277 seconds (4m 37s) and to 10% in 920 seconds (15m 20s). These values are good to keep in mind when trying to shoot down the ranker with a low grapes amount.
The only real problem that might slow you down a lot is autopromote: if rankers would be constantly autopromoting there won't be any targets to shoot.
Winning the round is very possible, although frequent vinegar throws might make you vulnerable in AH-ladder.