Here is a step-by-step guide on how to get the riposte achievement if you’re having trouble.
Before I continue, credit to this discussion for giving me a large chunk of this strategy
Step 1: Turn off weapons.
Step 2: go on maps w/out stage hazard and are flat like factory3 or the forest stages. This is just so the person attacking will be easy to block.
Step 3: Set HP to one. This makes it easy to finish off the opponent after blocking.
Step 4: Kill all except whoever’s getting the achievement + 1 other.
Step 5: Have the other person stand a good distance away. This is to make their movement easier to see coming when blocking so the block can be timed.
Step 6: When you’re ready, tell the other person to walk and punch constantly towards you.
Step 7: Block their punch, and punch them without moving. This is what most of the setup is for, as blocking is hard. Since their motion should be predictable enough it reduces how many tries it takes. Remember that blocking ‘recharges’ over time and becomes less effective the longer you use it so only block just in time so it is effective and has a wide range.
Step 8: Repeat steps 4-7 until you get the achievement. This can take a while, so don’t worry. Remember that even if you get the block and punch in, it still may not give the achievement. You are not doing anything wrong if this happens, there is simply a hidden parameter the devs made with the ‘immediately kill’ part of the achievement description. From what I can tell (and this may not be correct!) is that one wants to punch as near to the place where one blocked as possible as soon as possible. When testing, it didn’t work the times that the opponent and I moved after blocking, even though the punches were pretty immediate.
And that’s it! Good luck to all of you out there having trouble with this specific achievement, hope this guide helps. If there’s any feedback or anything I missed, please tell me.