Oompa said:
I know the best lesson is to kill them off
It isn't. Killing them off without actually showing them how to improve their tactics are better is like giving someone an F on a paper without explaining why they got it. If you want them to improve their tactics, I'd recommend a few things:
(1) Talk to them as players (not PCs) and explain that you think their tactics could be much better and that they're significantly handicapping themselves with their current tactics.
(2) Offer to give them some ideas for better tactics and run a one-off adventure or just a series of fights where they can practice those tactics and come up with others.
And, most importantly, discuss whether they are really interested in getting much better tactically. Everyone doesn't play D&D the same way. Your players, or maybe just some of them, might just prefer a game where their PCs don't need to be great tacticians to survive. And that's perfectly okay, as long as you're willing to and capable of running such a game.
What should be an good party, monster that they can fight and learn tactics? I plan on rolling open all night..
One of the best ways to teach a group tactics is to use those tactics against them. So I'd recommend hitting them with a group of enemies who are similar to them but significantly weaker, and have the enemies punch much above their weight through good tactics.
The party is..
9 wizard, 9 fighter, 9 paladin, 8 cleric, 8 rogue, 8 druid...
Going with what I noted above, maybe have them go up against a 6 person NPC party, consisting of a Wizard 7, Fighter 7, Paladin 7, Cleric 6, Rogue 6, Druid 6. If the players are as tactically weak as you say, the NPC party should be able to take them down through intelligent use of tactics.