I finally realized why the Kaiju Otachi and Leatherback were so much more effective against the Jaegers than their predecessors. We already know that the Kaiju have a hive mind, so they come to Earth with knowledge of all the previous Kaiju battles. The Masters were likely sending Kaiju through that could deal considerable damage to populated areas, while still being able to fight Jaegers.
When Newton drifted with the Kaiju brain, due to their hive mentality, the Precursors were able to see into his brain briefly as well. They saw that all three remaining Jaegers were stationed in Hong Kong, and that there were no more being constructed. Crimson Typhoon, the Chinese Jaeger, had continued to best Kaiju in battle due to its maneuverability, speed, and three-arm spinning blade technique. Cherno Alpha, the Russian Jaeger, had served for many years at this point, and had the heaviest armor of all the Jaegers, (which presumably had not been penetrated by a Kaiju).
Finally, there was Stryker Eureka, the Australian Jaeger and only mark V. As a mark V it was the newest, fastest, and strongest Jaeger. The pilot stated near the beginning that they had just beaten the record of most Kaiju slain as well. And if you saw that fight (the one in Sydney), you know that Kaiju stood stood no chance. This means that, even though Stryker Eureka had been in quite a few battles with the Kaiju at this point, the Precursors still did not know how to beat it.
The Precursors had learned from Newton that the three Jaegers they have yet to destroy are the only three remaining, and all are stationed in the same place. They send two Kaiju, each specially designed to destroy the three remaining Jaegers, and to get rid of Newton before he discovers anything else about them.
Crimson Typhoon had bested many Kaiju through its agility, speed, and third arm (seemingly unique among the Jaegers, thus making it inefficient to create a Kaiju specifically to counter this single Jaeger). To counter this, Otachi was designed to be smarter and faster than other Kaiju. To deal with the third arm, it was given a claw on its tail. This third claw was much more flexible than an additional limb, giving it an advantage over the Typhoon. It had seen its fighting style through the eyes of previous Kaiju, and as such had learned how to counter it. Cherno Alpha’s armor appeared to be stronger than any other’s, Kaiju or Jaeger. To counter this, they gave Otachi an acid weapon.
Stryker Eureka was the one they were unable to deal with until now. Because of that, they created Leatherback with an EMP specifically to deal with it. It also had the heaviest armor of all Kaiju up to this point, in order to make sure it was able to use its weapon.
When the battle starts, Typhoon and Cherno are out in the water by themselves, while Stryker is back by itself. Otachi was designed to take down Typhoon and Cherno, so it engages them while Leatherback sits back, waiting for Stryker. Otachi takes down Typhoon and is fighting Cherno when Stryker starts moving towards the fight. This is when Leatherback jumps out at Cherno; it does so in order to take Cherno down before Stryker arrives, and to make sure Cherno is unable to stop it from using its EMP. Once both Jaegers are down, Otachi heads towards the city to find Newton, and Leatherback uses his EMP to stop Stryker. He then begins circling the disabled Jaeger, studying it up close, likely for the Precursors to get as much information on it as they could.
When Gipsy Danger is lowered into the water behind Leatherback, you can see that it is surprised. After all, that Jaeger had been destroyed years ago. They were not prepared for a fourth Jaeger, and that may be the only reason they did not win this war right then and there.
The Precursors’ minds could not allow Newton to drift with another Kaiju brain and figure out how to get through the portal. They put into Otachi a secondary smaller Kaiju, one that was to come to life when it’s secondary brain chamber was breached (notice how there were no signs of additional life until they reached its secondary brain). This was their plan B if Otachi failed to kill him. They may have even designed Otachi’s secondary brain to destroy itself if it were to die. Unfortunately for them, the baby Kaiju plan backfired, as they ended up using its secondary brain as a replacement.
This is all my personal thoughts though, but the more I think about it, the more well thought out this movie’s plot seems.
tl;dr: Otachi and Leatherback destroyed Cherno Alpha and Crimson Typhoon, and disabled Stryker Eureka so easily because they were literally designed for it.
Leatherback turtles are named for their shell, which is leather-like rather than hard, like other turtles.
They are the largest sea turtle species and also one of the most migratory, crossing both the Atlantic and Pacific Oceans. Pacific leatherbacks migrate from nesting beaches in the Coral Triangle all the way to the California coast to feed on the abundant jellyfish every summer and fall.
Although their distribution is wide, numbers of leatherback turtles have seriously declined during the last century as a result of intense egg collection and fisheries bycatch. Globally, leatherback status according to IUCN is listed as Vulnerable, but many subpopulations (such as in the Pacific and Southwest Atlantic) are Critically Endangered.