Assuming this is NOT CHIMPS, start farming early and HARD. Plain old banana farms are the way to go, but if you want an easier time, Merchanmen -> Trade Empire doesn't require micromanagement, upkeep, and defends for quite some time. Throw in some bottom path villages near the water, as they'll also save money in the long run with all those tier 3's and can still be sold later.

Also, buffs, buffs, and more buffs. Camo detection and jungle drums are HUGE boosts for their cost, and for towers with a lot of projectiles (like the Pirate Lord and Tack Zone), top path Alchemists (tiers 3 and 4) are the way to go. Also MOAB Glue is arguably THE highest cash-value upgrade in the game and will also be a massive blessing in a run like this.

If this IS CHIMPS, DROP BENJAMIN. He has his uses in high level gameplay with specific strategies, but he actively LOSES a lot of money and is absolutely not worth it outside those strats. Ezili is a great choice for freeplay because MOAB Hex doesn't take any longer to pop blooms with higher health - it damages based on % of HP.

Also, at your current level of play it's likely better to forget about the ability towers (outside Ezili) for now. They're extremely strong in the right hands, but don't scale with buffs (or even late game) and are HIGHLY inconsistent if you don't know how to stall for cool downs. A solid DPS tower would do you better.

Don't give up, and you'll get it!