[Amos]

It should be:

#newmonster 2500
#name "Light Infantry"
#spr1 "./dir/LI.tga"
#spr2 "./dir/LIAtt.tga"
#desc "TBD"
#ap 12
#mapmove 2
#hp 10
#prot 0
#size 2
#str 10
#enc 3
#att 10
#def 10
#prec 10
#mr 10
#mor 10
#gcost 10
#rcost 1
#weapon 1
#weapon 21
#armor "Scale Mail Cuirass"
#armor "Iron Cap"
#end

I dont know what directory your sprites are at so I put "dir" instead. You need to insert directory thats in your /Dominions3/Mods dir.