I think you could even do this with normal blast using tblastx, if it is just a single query anyway. The NT database is also simply a huge nucleotide collection, and works fine for a few query sequences.

and i need to make a db for every assembly file that I have, which is unfeasable.

Why infeasible? Simply concatenate all assembly fasta files into a huge one and run makeblastdb, or make a blastdb for each assembly file using makeblastdb and combine them using blastdb aliastool. I don't see any problem with doing that. When doing this in bash by typing cat *.fasta > big.fna, the only thing that could break is that the commandline becomes too long.