[achieve, reach, attain, gain, approach, acquire.
accomplish, complete, finish] These are very difficult for me to explain. Sorry, but I'll have to leave that for someone else to answer for you.
predators to up to the grasshopper--There should be another verb here, like "Predators stalk up to the grasshopper" or "Predators move up to the grasshopper". The general idea is that the predators are approaching the grasshopper.
sea dragon is sea horse 海马？--Sorry, I can't help with that either...I have no idea what that is :/
you may be on to somethins. it means you maybe right?--It should be "You may be on to something" but yes, it can mean "you may be right", or "That might be a good idea". It depends on how it is used.
Sorry I wasn't really helpful, but I hope you can use the little information that I have