I must say this event's 6*, if needed same items as in the Japanese version(350k), will be quite hard for anyone without a melee/missle 6* which specifies at land attack.
I just used all my TP on 2 liliths, which is hard enough that I got a Borage in my inventory @lv113 to begin with(now lv122), and only got about 4700 gems, with help of multiple 6* allies with multiple 6*s. Currently I just met my 3rd.
and that's about 4 hours worth of TP, which, is still kinda more efficient than grinding and finding those low lv 4* bosses.
However, lower lv players are very likely to have tons of problems killing liliths within 3 hours, even if you do kill it within the given time slot, you have to have fun attention for quite a while for at least every 30 mins.
The event runs for about 11 days, each day you have 24 hours, and if you can just kill 1 every 3 hours(given luck and keep meeting lilith at every refresh, which is unlikely as my alt only met 1 so far), you only get about 8 liliths a day, 88 for the entire event, and 2000 gems for each grants you only 176000. Even if you some how gets really lucky and have an average of 3000 gems per kill, you only get 264000 gems.
And we all know that this did not take into account of ally assists(which a 5* player will likely have trouble getting continuous 6* melee assist).
So, unless you can get lilith down in 2 hours or less, and is willing to spend some potions, and is extremely lucky, this event's reward 6* seems out of reach.