1. Eliminate a player
2. Take an objective
3. Break an objective
4. Take a spoil
5. (nuclear) Remove your armies from a region you will probably be forced to nuke yourself next turn.
6. (team) Contact partner in order to make an advantageous reinforcement
7. (team) Contact partner in order to allow him to do an advantageous reinforcement
8. Capture a bonus
9. Break a bonus
10. Drop opponent to 3n-1 regions (11,14,17...)
11. (flat rate) Capture a region whose spoil is on hand
12. (fog) Block opponent's information to a critical action of yours.
13. (fog) Capture an irrelevant neutral giving the impression that you just conquered a bonus in the other side of the map
14. (trench) Increase your boarder line in order to prevent opponent from breaking through a critical area.
15. (automatic, chained) Block a line of communication through which opponent continusly reinforces misplaced 3s into action.
16. (automatic, chained) Open a line of communication through which you can reinforce misplaced 3s into action.
17. Engulf a thorn. Attacked region is the only one left "inside" your territory and opponent will wreck havoc if allowed to place his army there next turn.
18. Unblock third party. Open the rout to another player in order to attack with his big stack another player who is threatening to win the game.
19. (fog) Gain critical information about regions behind.
20. (escalating, fog) Any kind of bluffing-trying to seduce an opponent into an elimination attempt, based on wrong guessing of target's area positions.
21. (round limit) Last round's desperate attacks to surpass your opponent's army or creating a situation where a specific senario of conflict between two other players will lead to a win.
22. (team, round limit) Creating as much damage as possible against opponents when you are the weak player of the team and your army size does not count for the result of the game.
23. (escalating or any other case of cleaning up an area or an opponent) Make suboptimal attacks (mainly 3 vs 1 and 2 vs 1 but even 2 vs 2 sometimes) with not so much useful attackers when you need to "narrow" or "facilate" the advance of your main army in an otherwise dubious or uncertain elimination attempt.
24. Conquer a critical region where you want to reinforce in, be it an escalating block, a passing point to a game objective or anything else that matters in the game.
25. (specific maps killing armies) Use against opponents the spare army (armies) that are going to be wasted at the start of your next turn anyway.
26. (freestyle) Freestyle tricks. We can all imagine many situations where you completely bust opponents plans by interfearing with an otherwise stupid attack.
27. Irrelevant isolated battle (only for 3 vs 1 and 2 vs 1). The two regions are isolated from the game because of neutrals (for example the two south Australia regions in classic, when the upper two are neutrals). Making this 3 or 2 vs 1 assault can not hurt you in case you lose, but will provide a +1 region for you and a -1 region for opponent in case you win, FOR THE WHOLE GAME to follow.
28. (Specific maps) Bombardment. You have serious chances to hit too many regions, just with the same 3 troops.
29. (specific maps) One way assaults. Forever removing opponent's ability to use a one way assault route, thus trapping him in an unfavourable area.
30. (escalating) Encourage a favourable (!) elimination attempt by capturing an isolated region. Isn't it so nice when in a multiplayer escalating game you offer one opponent the chance to win the game (60% or 40%, really doesnt matter) but you in turn win the game yourself if he fails to do so?
You can bet I can find much more with some extra thinking. That was a funny list of me to do, but please do not miss my main point. It is wonderful when a cook thinks that "I will make this normaly unfavourable 2 vs 1 attack, because opponent's reinforcements from the south will be cut off in case I succeed, but the wasted troop in case I lose is not going to hurt my game because of its isolation". We should provide new players with some normal rules, but also encourage them for thinking creatively in order to spot the exceptions to those rules.