。
Lightning is prone to strike metal.
Literal
Thunder/lightning [topic-は] metal [to-に] fall-easy.
Note 雷 here means 'lightning' (the strike), not 'thunder' (the sound) — Japanese 雷 covers both, disambiguated only by context. 落ちる + に ('fall to / strike at') describes the lightning landing on a target. やすい ('easy to') is the same suffix as.