In the absence of any other further description a 50 kilowatt energy weapon is actually not very powerful. Probably just enough to destroy standard artillery shells and/or disable missiles at ranges of a couple of kilometers.
That being the case you have a number of options;
You can use larger/more well armored shells, perhaps even just high velocity armor piecing sabot rounds for example made from appropriately dense metals or metal/ceramic composites that are partially heat/shock resistant (At least for the second or so it takes to pass through the engagement envelope.)
Use fast moving smart weapons with sensors designed to lock onto the target (if its ground or sea based) just outside of the defense systems engagement range and which then shut down so they can't be damaged or destroyed when the defense system fires. Alternately they might launch some kind of protective 'shot' just ahead of themselves (like reactive armor) or otherwise put 'chaff' in between it and the beam. Depending on how long it takes the beam to burn through the interference the weapon may get close enough to detonate.
Then there's the option to blind/jam the devices sensors with your own energy weapons just prior to launching or firing (at close range) or as your rounds come in on final approach if fired from long range. (Perhaps every salvo fired contains one or more jamming platforms whose sole purpose is to target and blind defenses while the rest go bang!)
Or better yet, all of the above! Remember the defense system is short ranged so fast moving objects don't have to be able to survive for less than five seconds or so at most.