Sometimes you could choose a target that has cover, or that gives you advantage/disadvantage, with a Cleave. When its cover, simply add the cover to the creature's AC as normal. If it's advantage/disadvantage, use the passive modifier of +5/-5 instead of rolling. This way the game is not slowed down by making additional rolls.
When with a ranged attack you reduce a target to 0 hit points and there is remaining damage, you can choose another target within a 20-foot long 5-foot wide straight line directly behind the downed target and within the range of your attack. If the original attack roll would also hit the new target, it receives the remaining damage.
You can repeat this process until there are no more targets or there is no remaining damage.