your best bet is to mask off up to the line that you need to trim……I take it it's the abs hull.
the masking tape will stop the knife slipping.
take a sharp scalpal type craft knife..i use the swann morton orange type knives. and make some light inscisions along that masked line.take it carefully….and you DON'T need to cut through the plastic……
once you have made an inscision, you should be able to bend the plastic wast away from the cut, and it will ( loke glass) snap off along that line……..only trim the hull………leave the superstructure until you have put the deck on before trimming that to a level, and then come back to us.
neil.