There is no end in this mode but you can go as far as your computer allows if you follow my strategy.
1. To start you should add 2 monkey towers, upgraded, and aiming for the last balloon. This will keep them hitting 2 targets per shot and will let you save money for the next tower.
2. As soon as you have the money, add a third money with the same configurations.
3. Time will go on and as soon as possible, add one glue tower right near the entry to catch the balloons before they are hit and the glue will melt them before they reach the end.
4. As soon as you have the glue tower save for the bananas because without them you won't be able to reach level 100.
Now keep saving money for more banana farms and add more towers only when you really need them.
6. Towers that will help you in the middle game is the ship fully upgrades and the spike tower.
7. Keep upgrading the banana farms with all your money.
8. When you see the first MOAB coming it's time to stop upgrading the farms and to build as many rockets as you can.
9. Once you have enough rockets, just fill the screen with wizards towers and you will beat level 100.
Can you get to the level 100? If you do then no next level will be harder than the previous and you can go only as far as your computer allows. This is a great game that despite already having some years, it will continue to amuse you for several years to come or at least until the 6th version of the game is released.
Garen is in my opinion the strongest champion to play in the top lane. In the late game he can sustain so much damage that he can take out a turret alone..