can i use retreat to target a non friendly unit just to cast the spell and burn it? if i just want to get a card played and dont care if it fizzles. any ways?

No — you can't. Retreat's "friendly unit" restriction means enemy units are illegal targets (Rule 355.9.b), and you can't put a spell on the chain without legal targets (355.8). If your only units are gone, Retreat simply can't be played at all — so there's no way to burn it just for a "when you play a spell" trigger.

The closest edge case: if you do have a friendly unit, you must target it, and it won't fizzle — the bounce resolves normally ("silver rule": you do as much as possible, FAQ #7360). The spell only whiffs if your target gets removed before it resolves (e.g., something else bounces or kills it in reaction). In that case it still counts as "played" for triggers like Red Darius (FAQ #3712), but you can't engineer that on purpose with Retreat itself.