TLDR: Yes events are hard, for a reason, and YES F2p can complete events when you get to end game content level
I view these events as "end game content" and I participate up to the level I can when the achievable low level rewards are relevant to me. I like getting the smaller rewards along the way in the events, even though I don't have the saved resources necessary to complete them yet. If you think about it, if all these things were just easy to get, and anyone could get them, then everyone would have them, and there would be very little variation in champion collections when competing in the arena or comparing to clan mates in clan boss damage. The game would get boring quickly and then no one would be playing or working toward anything anymore....
When I mention saved resources, I mean that, even IF I was lucky enough to collect all the champions necessary to do the fusion, I would need to have saved up:
Lucky Champions in the Vault Ready to go:
Maybe they dropped from shards, and you just fused them, leveled and ascended them already to get some reward from a previous event. They didn't fit on a team, and you have no current use for them. This would ease the burden on the requirements coming up next.
Ascension resources:
120 Lesser affinity potions + 60 lesser arcane potions
128 Greater affinity potions + 81 Greater arcane potions
18 superior affinity potions + 9 superior arcane potions
Silver:
$300k-?????k required to fuse the epics and the legendary
Energy/Silver/brews/Upgraded Sparring Pit:
whatever combo you can do to get 12 champions from 1-30/1-40 and 4 champions 1-40/1-50
While I currently have NO WHERE NEAR those kind of resources, as I get to the final stages of all the dungeons and fill out my special teams, I see how I could easily have this saved up, even as a free to play player, when I am at end game content and all my dungeon teams are set, geared, and fully mastered.
TLDR: Yes events are hard, for a reason, and YES F2p can complete events when you get to end game content level