First thing I would do is fly it with the ailerons set to level, and see what that gets you. Clevis adjustment should allow for this, or if not, install new pushrods. For sure, you will find the model climbs less, or maybe not at all.

If it does still climb with downtrim on elevator, then you can try setting ailerons to a slight uptrim (maybe 1/16").

What trainer is this? How are your wings mounted (rubber bands or bolted)? Another fix might be to put in a bit of negative incidence (lift the wing trailing edge with 1/16" shim). Any possibility you have down-warp in the trailing edges?

Obviously you want to trim the wing so that your elevator trim is neutral for level flight.

10 degrees of downthrust?! and still climbing??!! Sounds to me like this model needs to be wrung out by a competent instructor.