Easy and way too exploitable.
Extreme difficulty walkthrough:
On the ship front, Scouts, Anti-Subs and Choppa's are all terrible, the easy path to extreme is exploiting the damage of Subs and Anti-Air.
On the weapon front, Flack and Airstrike are almost completely useless for how much they cost, Mines are only useful for a couple levels in the top path (but you can win easily without them).
Fog of war is such an insanely overpowered ability you shouldn't be spending research on ANYTHING else except EMPs, which are semi-OP.
For an easy win on Extreme the first thing you should do is buy riggers. You should have just enough to buy a 2nd scout before the battle starts. With 2 scouts and EMP you win the first battle easily with no loses for the +500 gold. Upgrade Oil Rig and buy 4 more riggers. Sell the Kamikaze and you should have enough money to buy 4 scouts before the next battle.
Fight the 10x scout battle and immediately retreat your scouts when they get under 5 HP (you'll lose but do some damage to them). Heal your scouts and upgrade all 4 to level 2, then reattack the 10x scout battle, same as before, retreat your scouts when they get below 5 HP (probably won't win this one either). Heal scouts DO NOT UPGRADE, wait out time, re-attack the 10x battle again and win easily.
Now upgrade Naval Base to level 2 and buy TWO scouts (yes you have plenty of money to buy more but save it.
Attack the 5x surface and 1x submarine level, win easily and upgrade Submarines. Sell ONLY 4 of your scouts (keep 2) and buy 6x Submarines and upgrade all 6 to level 2. Now attack the 10x surface ships and 1x Choppa level (thats why you kept 2 scouts). You'll wipe out the surface ships but the AI on scouts is total crap but they should do about 1/2 damage to the Choppa before having to retreat.
Retreat is good because at this point your riggers should all be level 4-5 and you should get MORE money from a retreat than the +500/+300 bonus money from winning a level in the 1st or 2nd time gives you.
Buy 3x Boffins and upgrade them to level 3 so they start getting research in. From now on you should be able to keep 3 EMPs in your inventory to use as necessary.
From this point if you feel like you are too weak to easily beat fights, just do a bit of damage and retreat all ships, farm more money from riggers (which incidentally will level them up faster and give you MORE money down the road), heal, upgrade a few ships, and go attack it again.
Go back and attack the same level again, make sure you do NOT win and retreat before the choppa dies, with your money upgrade all subs to level 3. Go back and win the level, now upgrade Oil Rig level 3, fill it out with Riggers, immediately upgrade them all to level 4.
Go attack the 10x Scouts, 1x Anti-Sub, win easily (subs should be 1 shotting enemy Scouts at this point).
Upgrade Oil Rig to level 4, buy riggers and upgrade to level 4.
Go attack the 5x surface, 5x subs level, use an EMP when your subs are shooting at each other, win easily.
Now upgrade Naval Base to level 3, buy 2x more subs (should have 8) and upgrade all to level 4.
Hit the 20x Scouts, 5x Anti-Sub level, win easily (use an EMP or two if you are going to lose subs).
Upgrade AA Gun, buy 4x AA gun and upgrade all to level 4. Attack the bottom path battle (accidentally clicked it while writing this so I don't know the exact breakdown). Use a couple EMPs if necessary, win easily.
Upgrade Naval Base to level 4, buy 2x AA guns and 2x Subs and upgrade all to level 4, giving you a total of 6x AA guns and 10x Subs.
From this point the strategy should be incredibly obvious, hit levels you think you can beat, alternating between the paths, retreat after using 3x EMP and doing some damage if you are going to lose ships or if you want to farm money, then go back and win. Stay away from the top levels when you hit the 10x enemy big sub level until you have level 10 all ships, the big submarines are really annoying to kill (and Anti-Subs do jack against them, which is why I don't buy them). Once you have a couple Fog of Wars they become dogmeat to your own subs, though.
As you win levels, upgrade your War Room all the way to level 4 and buy Boffins next, upgrade them to level 4-5 on buying each one, and when you have Level 4 War Room, next upgrade Fog of War.
Fog of War is an unbelievably overpowered ability that prevents the enemy team from attacking for several seconds while you wipe out most of them. 3x fog of wars on any level with 10x Subs and 6x Anti-Air all level 10 is an auto-win (even on the final boss stage).
When that's done upgrade Tesla Ship and buy one (it plays havoc on the enemy, then start upgrading your Special to level 4, getting 3x Teslas total. At level 4 upgrade Repair ship, and buy 1x repair ship.
Now just go win the game.
Bam, extreme achievement unlocked.
