there are a few tricks you can use to beat eternia, if you spend a little more time in this forum you will surely stumble upon them.
do note tho that it is not intended for eternia to be easily beatable. he is sopposed to be the ultimate challenge afterall. most games will need several pro people with good teamwork to win eternia. donator stuff does help, but really the single most OP thing you can get your hands on is the elemental stone itself! convince a few people who already have beaten eternia to join you and pool the elem stones of everybody on 1 hero. if you got good income you can kill him 1v1 with the buffed hero.
there are methods that dont rely on using the OP elem stone (if there werent then nobody would have it lol).
you really need to figure out how you will play endless waves 1-9 first if your base cracks before eternia it's no use. also many ways to beat eternia require special preparations (eg massing high levels walls some place to distract eternia and help the hero survive)