。
I felt it was a comparatively difficult jigsaw puzzle.
Literal
Comparatively difficult jigsaw [copula-だ][quotative-と] felt.
比較的 'comparatively, relatively' is a formal adverbial, common in reviews and assessments. ジグソー is a katakana loanword, short for ジグソーパズル (jigsaw puzzle) — another example of clipped loanwords. 感じる 'to feel, to sense' takes its object with を for direct perception or with と for a felt judgment (as here). A reflective comment from a puzzle enthusiast.