Now my problem is the tip of the leaves become brown and dry, it doesn't
affect the complete leave (most of times), but they look ugly and affect the
look of the tree (and I don't know how far also it's health)

Maples & other thin leaved species are very susceptible to leaf scorch. You
don't want to give it too much shade, as that will defeat some of the other
bonsai processes. Try to get hold of a product called Pro-TeKt from Dyna-Gro,
or the chemical Potassium silicate. This works wonders for me, & we have very
dry windy summers. Meanwhile, see if you can erect some sort of windbreak
without further reducing the amount of sun. Make sure the tree is adequately
watered without overwatering.

Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)