Victory in Ljubljana on Sunday will guarantee the Scots second place in Group F and, crucially, a spot in the playoffs as one of the eight best group runners-up.
Despite being out of contention for Russia, Slovenia have not conceded a goal at home during qualifying.
"This is the hardest one now," Strachan told BBC Sport regarding the task facing his team.
"It's not just (quality) at the back. It's all over the pitch. They're physically strong, technically good.
"From what we've seen so far, it's similar to a 4-4-2, they've got good players up front, they make sure there's very little room in the middle of the park."
Scotland last reached the finals of a major tournament in 1998 when they qualified for the World Cup in France.