1.  Satan is a very subtil or crafty enemy
Let us beware! Our first introduction to satan in the Old Testament scriptures is in relation to his SUBTILTY
This is a warning to the human race. SUBTILTY is the means by which satan deceived the first parents and the rest of  the human race:
Now the serpent was more subtil6175 than any beast of the field which the LORD God had made. And he said unto the woman, Yea, hath God said, Ye shall not eat of every tree of the garden? – Genesis 3: 1.
But I fear, lest by any means, as the serpent3789 beguiled 1818Eve through his subtilty 3834, so your minds should be corrupted from the simplicity that is in Christ.

For if he that cometh preacheth another Jesus, whom we have not preached, or [if] ye receive another spirit, which ye have not received, or another gospel, which ye have not accepted, ye might well bear with [him] (2 Corinthians 11: 3 & 4).

2. What do the Old Testament scriptures mean when they  say that satan is subtil?

a. The subtil plot schemes to kill others

The Hebrew word for subtil is `aruwm (H 6175).  CRAFTY is the synonym used in the Old Testament scriptures.

Related scriptures in which subtil  (`aruwm/H 6175) is used show that the crafty are those who plot schemes in order to trap and kill other people.

Remember that satan plotted to kill Adam and Eve and was successful in his endeavour:

He disappointeth the devices of the crafty6175, so that their hands cannot perform [their] enterprise
He taketh the wise in their own craftiness: and the counsel of the froward is carried headlong
They meet with darkness in the daytime, and grope in the noonday as in the night.
But he saveth the poor from the sword, from their mouth, and from the hand of the mighty – Job 5: 12 – 15.
b. The subtil speak with clever deception
The scriptures describe the tongue of  the crafty or the subtil. Satan craftily used words in order to deceive Adam and Eve.
He perverted the truth and deceived Adam and Eve into believing his lies:
For thy mouth uttereth thine iniquity, and thou choosest the tongue of the crafty6175   (Job 15: 5).
Your sins are telling your mouth what to say. Your words are based on clever deception (New Living Translation).