Look at what you have said. You have not explained how He would do this. You just restated what quatona said.
I did explain how: turn up physically, demonstrate that he is indeed God by performing a few miracles - this is what Jesus did, after all, and many came to believe in him because of this, even those who were initially opposed to him and doing evil, according to the Bible.
Why use a method that caused terror and pain? What method of causing one's bodily processes to cease does not carry with it some sort of terror or pain?
A gunshot to the back of the head with the person being completely unaware that they were in any danger - no terror, no pain. I am sure that God, supposedly being a little more intelligent than me, could come up with another method - for example, complete instant disintegration of the brain.
God did what He did so that they who were wicked and were judged might serve as an example and warning for all who were to live after them. The sin was so great in the earth and it had become so corrupted that God knew that we sinful human beings, prone to forget what history has taught us, would need a vivid reminder of the consequences of sin that we could look back on and He loves us so much that He caused the flood to happen for our benefit. God wants as many to repent and be saved as possible and He sometimes has to scare us and frighten us into waking up out of our deep sleep so that we might, if for no other reason than fear of judgment, seek and pursue righteousness and repentance.
By doing horrific things to innocent creatures - all the animals were innocent. The lesson I take out of this is that if you are innocent you may have horrible things done to you by God just to send a warning to people who are not innocent but might repent of their evil.
And we would not need a historical event to remind us - and historical events are not very good reminders of things, as you said - if God walked around with us all the time, giving us tours of Hell and Heaven, and pointing out the consequences of our actions.
And, as quatona said - and, yes, I repeat what he said because you do not really address the things that he says - God could think of no better method? This is an omniscient being. Inflicting suffering on innocent animals and children was the absolute best he could come up with?