Here is a more general result.
Theorem: Let (K,||)(K,||) be a non-Archimedean normed field with completion hatKhatK. Let mathcalL/hatKmathcalL/hatK be a finite separable extension of degree dd. Then there exists a degree dd separable field extension L/KL/K such that LhatK=mathcalLLhatK=mathcalL.
In particular, as long as the completion of KK admits a separable field extension of a certain degree dd, so does KK itself, necessarily of the form K[t]/(P(t))K[t]/(P(t)) by the primitive element theorem. Moreover, as long as KK has characteristic zero and carries a nontrivial discrete valuation, it admits finite separable extensions of all finite degrees.
For a proof of this theorem using Krasner's Lemma, see Section 3.5 of
http://math.uga.edu/~pete/8410Chapter3.pdf
When the norm corresponds to discrete valuation vv (e.g. ||=||mathfrakp||=||mathfrakp
the mathfrakpmathfrakp-adic norm for a prime ideal mathfrakpmathfrakp of a number field KK) one can get away with less: by weak approximation, there exists alphainKalphainK with v(alpha)=1v(alpha)=1. For any positive integer nn prime to the characteristic of KK, by Eisenstein's Criterion the polynomial tn−alphainK[t]tn−alphainK[t] is (separable and) irreducible even over the completion hatKhatK, so is certainly irreducible over KK.
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