Learn Spanish (Latin America): La estrategia se diseñó contra el competidor. - The strategy was designed against the competitor.

While fue diseñada is technically correct, spoken Spanish heavily prefers the "passive se" structure (se + 3rd person verb) when talking about inanimate objects like la estrategia and the person who did the action isn't mentioned.

Why is diseñó in the preterite tense rather than the imperfect (diseñaba)?

We use the preterite here because the designing of the strategy is viewed as a single, completed event in the past. If you wanted to say it "was being designed" (an ongoing background process), you would use the imperfect.

Could I say en contra del competidor instead of contra el competidor?

Yes! Contra and en contra de both mean "against" and are generally interchangeable here. Contra is just slightly shorter and more direct. Just remember that if you use en contra de, you must combine de and el into del.

Is it normal to put the subject first (La estrategia) in a passive se sentence like this?