How to kill 600 tier unit?

Your question didn't specify the tier but for example if you need to kill tier 5 units then you need to attack a player enemy (a real player not in your current alliance) who has troops of at least that tier.

Check the training camp to see what level of building each tier is unlocked at. eg Tier 5 unlocks at level 13, Tier 6 at level 16 etc.

Find enemy bases that are at least at that level and there is a good chance they will have the right troops. You can of course scout the enemy before attacking to check what troops they actually have.

Then attack the bases until you have wounded or killed enough troops.