As requested here I am creating this thread to tell you how to S-TEC someone with ease.
The idea in S-TEC'ing is to make your opponent run out of cards and to do this you make your opponent fuse from their hand. You will get duelists who will fuse easily (Villager 1, Mai, Rex Raptor etc..) and duelists who don't fuse so easily (Pegasus, Heishin 1st, Seto 2nd etc..) and the basic way to make your opponent fuse is to play 1 card face up that can beat any of your opponents cards, they will now play a card face down in defence mode. Don't attack that card but play a monster card face down in defence mode, now the computer should fuse, if not, attack the monster they just played and repeat until they fuse.. and when they fuse just keep picking off every card they play until they run out of cards and you will get an S-TEC rank.