1 Ethical rules need not be simple. They can be loaded with complexity. All they need is a ground, a moral property, and universality :
For every x, if Gx, then Mx : in every situation in which you are life-threateningly attacked [ground], in that that situation retaliate in self-defence [moral property].
Now that is a simple rule (not one I'm sure I'd accept) but it's clear from the logical form of a rule, just outlined, that the ground could be a monster conjunction and the moral property extremely complex. (I outline a modified logical form in 3 but it does not affect the substance of the argument.)
2 '"You must try for best outcome" is not a rule? What is it then?'
For every x, if Gx, then Mx : For every action, if that action produces the best outcome and is one you can possibly do, then you must try to do that action and hence try for the best outcome.
I'd call that an ethical rule.
3 I have fixed on the type of rule most relevant to your question but it's possible to have a universal rule when, for instance, there are plural and rivalrous values :
For every x, if Gx, then presumptively Mx. The presumption can be defeated if in a particular situation for action Gx grounds, for contextual reasons, a different moral property from M. That is to say, if a different moral property has priority in that specific situation. ('Whenever you make a promise then you should keep it' : in the real complexity of life a presumption is implicit. If the promise is to return a book, but returning the book would prevent you from saving an innocent life, then the presumption that you should keep your promise is defeated by the morally more significant consideration of saving an innocent life. The - to my mind - omnipresence of defeasible presumptions makes ethical rules merely rules of thumb, extremely rough guides, in moral deliberation and action. But that's another story.)