Learn English – Is it ok to say “I want to take showers / 2 showers now”

Even now has two meanings

• with a verb in the 'simple' construction it means ‘still, in spite of [some circumstance]’

  Even now, after all he had learned, he believed in her innocence.

• with a verb in the progressive construction it may have that meaning, or it may mean ‘at this very moment’

  He is pleading even now, even as we speak, with the Prime Minister to pardon her.

But even now is not used with a perfect construction in either of these senses.

I suspect what you mean is one of these:

Even now the boy's body is being devoured by disease.
Right up until now the boy's body has been devoured by disease.

In either case, the adverbial phrase is a sentential modifier and may be moved after the first auxiliary (is/has), after devoured or to the end of the sentence. It's permissible to move it after the second auxiliary (being/been), but I find that pretty clunky.

This use of see is not idiomatic in Anglo-American English. The corresponding minatory phrase is

I'll get you!

This probably represents an underlying “I’ll get you back” = “I will repay you for this injury”.

That idiom suggests a line that Arnold Schwarzenegger's Terminator films have made into a paradigmatic threat:

I'll be back.

It may also be possible that “I’ll get you” represents “I’ll get you where I want you” = “I will cause you to occupy an unpleasant situation in which I would like to see you”. I suggest this because there is an old AAE idiom, little used today but once popular in melodramatic dialogue, which used see in this sense.

I'll see you damned / in hell / hanged / in jail!

This does not imply literal presence when the addressee is in the named predicament, but gleeful anticipation of that situation. There is a discussion of one common (and still current) variant at ELU.