I have lillia on battlefield A with smoke and mirrors hidden. Opponent attacks A. I use smoke and mirrors to swap lillia with a sprite in base. When lillia moves and spawns a sprite, I react with en garde to the trigger, giving the sprite that lillia spawned which is alone +2 might first, then move the base sprite over for a total of 5+3 might. Is this correct?

Almost — the end board (a 5-Might and a 3-Might Sprite at A) is reachable, but not the way you describe: you can't En Garde "the Sprite Lillia spawned."

Two problems with that target. First, when you play En Garde in reaction to Lillia's move trigger, the token doesn't exist yet — targets are chosen at finalization, and the trigger hasn't resolved (Rule 352.4.b). Second, even once it exists it won't be "alone" at A, because Smoke and Mirrors already moved your base Sprite there, so the new token would only ever get +1.

The line that actually gets you 5+3: after Smoke and Mirrors resolves (Lillia → base, base Sprite → A), Lillia's move trigger goes on the Chain. Play En Garde on the swapped-in Sprite in reaction — at that moment it's the only unit you control at A (Lillia has left, the token isn't out yet), so it gets the full +2 (1 + 1 alone) and becomes 5 Might. Then Lillia's trigger resolves and plays the 3-Might Temporary Sprite there. This is the same pattern confirmed in FAQ #12488 (En Garde resolving before the token trigger is what locks in the "+1 alone" bonus).

If you let Lillia's trigger resolve first and then En Garde, the target is no longer alone, so it only gets +1 — that's the line to avoid.