As HRE Emperor I can't give myself right to vote. So any special tactics for this achviment? Other than just killing+releasing Mainz?
Play as deutsch minor and ally with the emperor. Then annex electrr, and wait for the time when the emperor makes you an electer.
The easiest way is with Austria, either PU Bohemia very early or wait for the PU mission, then fast forward 50+ years to inherit them.
Longer way is to play a theocracy minor and keep knocking out other electors. Theocracies are the most likely to receive electorship since they are completely unable to be valid candidates themselves. Republics 2nd most likely. AI prioritizes them above all else I believe, monarchies are highly unlikely to receive electorate whilst other types have at least non-hatred relationships with the emp