How to design index for faster deletes
Bit akward, I can't see how to comment but I wanted to ask why your condition isn't:

delete from ssn_sdo
where       
       art_id=@art_id 
       and skl_id=@skl_id
       and (@level = 0 or (@level = 1 and tip=@tip) or doc_id = @doc_id)


Not sure that helps at all, before or after you add any further indexes, but it'd interesting to see the executions plans (actual) and see if any benefit is realised.