Japanese spindle "tree" is a common name for Euonymus japonicus, technically an evergreen shrub. And like many broadleaf evergreens, it can be cut back hard.......to the ground or close to it if you like. Do it now or no later than early spring and it will reflush rapidly with new growth.

i dint really understand what the problem is.. the pruning rules dont change due to the angle of the trunk ...

just prune it up the way you want.. and if you dont like what you end up with.. you can do what gal said next year ..

when i had garden tours.. the things peeps like the best.. were the unusual.. this plant is unusual.. so either buy into that premise.. or do something about it... or be done with it.. its all up to you ...

ken