This event is possible with a few 60 heroes, but definitely no weaker than that. Idk if it’s even possible to finish with mostly 60’s. All I know is I passed it with a 3293TP team, but it took patience, the right items, and making sure MN never went off. Make sure your specials are charged before every stage, bring two healers if you’re struggling with the mobs, save your items til the boss stage and unload accordingly, save as many diamonds and/or bombs on the board [for the boss stage] as you can, use AoE and/or splash hitters (the splash hitters will be able to hit everyone at once), make sure to include some solid synergies (attack buff, defense down, etc), and again: Mother North cannot go off. Any stage with Mother North in it requires either a mana controller or timestops. If you let her go off even once, you’ll be going back and forth with her all day. Believe me, it sucks; don’t put yourself through that
Helpful items for boss stages: arrows, axes, bombs, dragon attacks (lots of cumulative damage). Mana pots for your mana controllers (big 100% one preferred), or time stops. Could bring in a health pot, in place of one of those other items, if you’re worried about not surviving the first stage long enough to get your specials charged (especially your healers).
There’s still about two weeks left, so I’d suggest going for it. My TP was over 700 less than what was recommended, but I did it. You might not be able to finish the event with a TP lower than 3k, but you might as well try. Keep pushing, and don’t give up; maybe you’ll be surprised at how far you get. Plus, it’ll give you some idea about how much stronger you need to be for the next seasonal event. Good luck