Toy Soldiers HD

This guide will focus on what is required to earn General rank, not how to beat each individual level. It will also include how to beat the final three tough Invasion levels on casual difficulty, thanks to an invaluable exploit.

To earn General rank on a level, you simply need to ensure you beat the battle with your Toy Box at 100% health. Amount of cash earned or held, units killed by you personally do not matter. Just Toy Box health.
Also, you only need to earn General rank on one difficulty per level. You don't need to beat the levels on German difficulty. You should play on Casual difficulty throughout.

You must beat all numbered (1- 18) campaign levels for the achievement. None of the survival missions count.

If your Toy Box loses health due to enemy troops entering it, you can replenish health by aiming an artillery gun in the direction of the enemy flag base and pressing 'LT' to launch troops, which will then run to the enemy base and replenish health for you. Remember the enemy base is guarded by two machine guns and sometimes mortars, so destroy those first with your artillery, tanks or planes.

INVASION LEVELS EXPLOIT

Due to a bug, the last three (16 - 18) Invasion levels can only be played on German difficulty, which makes earning General rank extremely tough.
There is an exploit to force the game to load these levels on Casual automatically. Note that this process should be a last resort, as it is very laborious and requires you to reset all your save data.

I did this all in one run just to be safe.

First, delete your save data. This will reset all your rankings on every level, your unlockable upgrades and your collectibles so be warned.
Alternatively, you can disconnect from the internet, delete your local save, and then delete THAT save after you get the achievement, so you keep all your original progress.
Otherwise, you'll need to play through every level again, all the way until level 15.

Now start a brand new campaign and play continuously to level 15 - The Last Offensive and stay on Casual difficulty.

Beat the level and choose to 'Continue' at the end screen. This will automatically load up level 16 - When Toys Collide on Casual difficulty, instead of the set German difficulty.

You'll now notice the level is extremely easy compared to on German difficulty. Beat the level as normal and ensure you keep your Toy Box health at 100%.

Once the level ends you'll be presented with two options: to 'Retry' or 'Main Menu'. Select 'Main Menu'.

From the main menu, select 'Play Toy Soldiers', then 'Campaign' and then 'Continue'. This will load up Level 17 - Not of This Toy Box, again on Casual difficulty. Beat the level as per the last level and again select 'Main Menu' at the end screen.

From the main menu, select 'Play Toy Soldiers', then 'Campaign' and then 'Continue'. This will load up Level 18 - Revenge of Toyzilla', again on Casual difficulty.

This is the hardest level on the Invasion missions, and even on casual you need to manually tend to the infantry and cavalry waves. I would deal with them with a Level 3 machine gun by the entrance to the Toy Box.

Throughout this level focus on planting level 3 mortars on every spare small emplacement, a level 2 anti-air tower halfway along the road from the Toy Box and Level 3 artillery on every spare large emplacement. Use the P-51 to wipe out aircraft quickly and whittle down infantry waves. This will give you plenty of cash to build those towers.

Once RoboBob arrives, attack him early with an artillery next to your Toy Box and gradually shift to the next artillery guns along the main road as RoboBob moves along. On his third form, he will become a flying head; this can easily be destroyed by manually using your level 2 anti-air.

Once he's destroyed, you'll have beaten every level on General rank and also earn the 'Mr Roboto' achievement too, if you hadn't already.

Here is a video from magikman79 who confirmed this works with his own defence strategy:
I wish I had found this out before beating the level on German difficulty first

Any feedback please comment and I'll amend the guide if required.