Good initiative on the -using genius to fight- part.
Since we are outnumbered when they shuttle in their hordes of volunteer soldiers who are only too happy to die, we should use unconventional means of warfare, to which they are not accustomed or prepared for. I am speaking of course, of asymmetrical warfare. Everyone should read up on what it is, and also I encourage everyone to study ancient and historic battles, which were almost always won by some innovative way of using the terrain or something totally unexpected in order to win.
Guerrilla warfare will also be one of the biggest advantages we will have, and everyone should know how to do it.
Forcing an army to cross a chest-deep river on foot, will make them just as disadvantaged from cold and muscle stiffness/contractions/psychologically run-down today, as a thousand years ago. Do not forget this...
Also remember the basics of warfare. Read Sun Tzu, etc.
The chinese have the most experience holding off hordes of soldiers. They used septic calthrops in the forests, exploding shrapnel-balls, scrap-iron launching mortars, etc.
My people have used landslides, avalanches of felled logs, floods, trickery and deceit to their advantage for over 1 500 years in THIS location...
Today some of my tricks would be a can of black powder with a box of nails in front and a soilid steel plate backing, and a fuse, or a tripwire connected to a simple flintlock and a pile of gunpowder, or really pointy and small calthrops with feces on the tips, hidden under leaves in the forests or wherever, chlorine bombs, "log-slides", night vision and silenced weapons, silenced and sufficiently powerful firearms to be body-armor penetrating (when lacking real armor-piercing bullets), a pack of really vicious and free-running dogs to take the edge off an attacker plus act as a warning system, etc. etc.
setting ambushes at choke points, blowing up bridges and roads, scorched earth tactics, caching, sniping/sharpshooting, picking an army apart man by man until morale is devastated and the muslim whore-sons start to question their beliefs after all their comrades have died without ever seeing who is attacking them even once,... using their dead as boobytraps, etc...
hit and run tactics, and much much MUCH more...