In order to beat a larger opponent you have communicate well, play smart, and rely on your preperations to win. Even if you aren't that big you should still have good defense for your size, as well good thieves, and wizards. A couple of wizard and thief specialists help too. Also you've got to play smart.
Ok, so what you do you do to convince these big bullies that they selected the wrong target? First of all gather information:
Crystal Ball, Crystal Eye, Survey, Infilitrate: Find out all the key info about your opponents. How many boats and wizards do they have? Can someone in the kingdom hit them? How are their thieves? Good, bad, or indifferent? Has their kingdom been fighting with anyone else that you may be able to message and get to ally with you? Post this information on your forum. Know your enemy.
Population Killers: Use a combination of fireballs, kidnapping, and arson to quickly cut into one of your enemies population. 3 or 4 provinces can deeply cut into a province's population. Once you get their population down keep continual storms on them. That will keep them from growing back. Their income will be small and they won't be able to draft any new soldiers. They will quickly be sorry they ever got involved.
Where Did My Boats Go? Sinking Boats is a great tool. Most people build just enough boats get by. For example if it's a Faerie with 10k swordsmen, you can almost bet they will only have 110 boats and the minimal amount of shipyards available. Sinking their boats makes those swordsmen useless and makes them frustrated. It's very difficult for them to replace those boats and since there is so much of a drop off for hitting smaller targets you've set them way behind.
Amnesia: Another attack from Hell. I once personally hit a guy for 100k points of Amnesia in just two days. This hurts there NW, reduces their effectiveness and trust me, they are counting every dollar it cost them in science that they lost thru those Amnesia spells.
Why are my troops leaving me :( ? Rob Granaries, Vermin, Drought, and even raze attacks (destroying some farms) can severely cut into a provinces food supply. The best time to nail someone with these attacks is when they are very low on food (duh :) and when they will be offline for a while. If you can establish a pattern (maybe a 24 hour bonus gatherer) and you hit them at an opportune time you can do some real damage. You'd also be surprised at how many people hardly look at their food totals. If they are really low to begin with you may be able to slide a drought in there and have them not even notice they're out of food. They may not even figure it out for two or three days if they are slow on the uptake. Meanwhile you've done horrible damage.
Misc Annoyances: Riots can be good to cut into income. Nightstrike and Nightmare can be useful if they are still relying on soldiers heavily for defense for some reason or to simply to cut down the troops the can train. Rob Vaults and Rob Towers are always nice to add to your resources at the expense of your enemies. Lightning Bolts probably aren't worth it unless they have a ridiculous amount of runes. Half the time you lose more runes casting the spell than the spell destroys. Mystic Vortex is just too expensive to mess around with unless you have huge amounts of resources. No need for me to even go into the defensive spells. Cast em if ya got em :)
Once you get these attacks going, communicate. The province whose population got wiped out? Make sure you keep storms on him 24/7. Don't attack that same guy with droughts because it's a waste of time. Make sure that you are focusing on all the people that are attacking you, largest first and the regent being public enemy #1. Trust me if the regent has no boats to attack with, a tiny population that you won't let grow, and is losing thousands of points of science per day thru amnesia, he'll be very interested in talking peace. The more you hurt the other large (influential) provinces on the other side the quicker they'll be agree. 7 or 8 thief and mage oriented provinces with good defense are enough to make any kingdom in the world back off if they use their forces wisely. Good Luck to you in fighting off any bullies that come your way....AF