That is the choke point for basically every player.
EOC Chapter 3 is what keeps you from 99% of the game's content.
The tactic is to not go full out on the sir seal and kangroo at the start, just stall them with Wall Cats and kill all the Those Guys. Once you build up enough cash, spam Dragon Cats along with Whale, Titan, and other powerful attackers. Make sure to keep spamming cheap units like Macho Cat, Wall cat, brave cat, and such