Learn Russian: Он месяцами мучился от сильной боли.

The bare instrumental plural of time nouns (like месяцами, годами, часами) is used to emphasize an extended, continuous duration, often with a feeling of frustration or tediousness. It translates best as "for months on end" rather than just stating a neutral timeframe.

Could I just say несколько месяцев instead of месяцами?

You could, but it changes the tone. Несколько месяцев is a neutral statement of fact ("for several months"). Месяцами adds emotional weight, emphasizing how excessively long and agonizing the period felt.

Why is "from severe pain" translated as от сильной боли instead of using the preposition из-за?

The preposition от (+ Genitive) is the standard choice for expressing the direct physical or emotional cause of an involuntary state or reaction. When someone suffers, cries, or shakes from pain, illness, or joy, you use от. Из-за is typically used for external, circumstantial reasons (like being late because of traffic).

I see боли ends in -и. What is the gender and dictionary form of this word?