Learn Turkish: Tanıtımın aksine, bu iddialı telefon batarya bakımından zayıftı. - Unlike the promotion, this ambitious phone was weak in terms of the battery.

The word aksine (unlike / contrary to) acts as a postposition here. When used with a specific noun like tanıtım (the promotion) or a pronoun, it requires that word to take the genitive case. This forms a possessive construction, literally meaning 'to the contrary of the promotion'.

Is iddialı literally translating to 'ambitious' for a phone?

Yes, iddialı literally means 'having a claim' or 'assertive'. In Turkish, it is frequently used to describe high-end or hyped products that boast impressive specifications. For a phone, it means it is marketed as a top-tier or flagship device.

Break down the phrase batarya bakımından. How does it mean 'in terms of the battery'?

It uses the noun bakım (point of view / perspective). We add the 3rd person possessive -ı to link it to batarya, and the ablative suffix -dan (with the buffer n). Literally, batarya bakımından means 'from the point of view of the battery'. It is the standard way to say 'in terms of' or 'regarding'.

Why is the past tense ending -tı instead of -dı in zayıftı?