...from nothing but a box of pathfinders and some savings. What should I buy? How should I play? Help??
Well you have to avoid melee at all costs, and make use of firepower.
Get some Kroot. They are the cutest little guys. Also they have Astartes Strength in melee.
Riptides. Multiple riptides. Unless these knights on the table or other super heavy. Its gonna be a hard slog to bring them down. They're also classed as elites.