I would recommend training core activation, deceleration, VMO activation (vastus medialis oblique fibers, but pretty much vastus medialis in general), the motions of all joints of the legs, proper absorption/dissipation/redirection of contact forces (bumping with other players), etc. If the player lacked power, I would include olympic lifts, box jumps/plyo (up mostly, down only from knee height or less), etc.
There are a number of other things that I might include depending on what I observe and assess in the client. I usually don’t give out advice without more information, so use this along with your judgement and own assessments of the client.
the first thing that comes to mind is SAQ training (speed, agility and quickness). There is actually quite a bit of information out there specific to Lacrosse, and this one looked good to me http://www.athletesacceleration.com/lacrossespeedtraining.html