I consider assassination any tactic which extends your reach with the specific objective of killing the enemy summoner in their own territory, rather than say clogging/destroying walls. Both of your examples are Assassin tactics.
1) You're playing as Natazga. You move your big units forward into the midfield, destroy enemy walls, and then proceed to kill their summoner. This is different because your primary objective was the walls, not the Summoner. This tactic is characterized as "Aggro" rather than "Assassin."
2) Saturos is marched up to the midline. You drop a big champ, move it 2 spaces, and kill him. This is not assassination because you didn't have to go into his territory to kill him. This tactic falls under "Defensive" rather than "Assassin."
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