。
She's good at slipping through the rules.
Literal
She [topic-は] rules [object-を] sneak-through [nominalizing-のが] is-skilled.
規則をかいくぐる is wonderfully pictorial: かいくぐる ('to sneak through, to slip through, to dive through') is itself a compound — 掻い ('scratch, tear') + 潜る ('dive, slip under'); together it describes a fast, evasive motion under or through something. Applied to rules, it captures the act of finding loopholes — technically not breaking, just slipping past. The X が うまい frame ('is skilled at X') uses が because うまい is an evaluative adjective treating its target as the grammatical subject. The の nominalizes the verb phrase so it can take the が-marker.