For Wave 28  you want to be on your tower boost monk tower boosting the dsts/lts and upgrade your defenses if you can while tboosting since it helps for the later waves try to keep mana for as long as psossible about 400-600 enemies in the wave your tower boost will go down but dont worry just keep getting mana and upgrading and reactivate your tower boost as soon as you get it back and you should be good for the rest of the wave just try not to run out of mana near the end as there is just ogres and 1 might get to your dsts/lts and kill them and you will loose just keep upgrading and tower boosting till wave 30 and on wave 30 u want to bring in apprentice and 2 gaurdian summoners and keep tboosting on wave 30 u dont have to upgrade just focus on keeping your tower boost up aslo if youre minions get low run near then with tower boost just not to close as harbs or ogres might kill you. Hope this build works for you 

Hint 1: if your minions are too far up the stairs a harbinger will hit them from where the other hint 1 is 

Hint 2: try and max this mage and get your minions to 1 star becuase i found if theyre not upgraded to 1 star they might die to the ogres and harbingers