what is the grammatical breakdown of this sentence?
よし = 良い = Good ==> Okay!
かかってこい = かかる and くる ==> かかる and こい! (in an imperative mode).
かかる can be spelled in many ways – 係る (regarding), 掛かる (hanging), 架かる (bridging), 罹る (suffer from), 懸かる (on the verge of / at risk) etc. However, かかる in かかってこい！ means “something unpleasant things/difficulties/challenges are heading to your place”. The image is like a tackle in a rugby game or a zombie chasing after you. Another set phrase with かかる is 迷惑がかかる (a nuisance to others).
かかってくる is a frequent combined verb. Don’t sweat it so much. Just memorize it.
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