You'll have to attempt all three and get to that level. I've played long enough that I usually manage to play through all three to 1200/1400/1600 in one hit You used to have to do each tier one at a time - which was a real drag ;).

BUT early on what you'll find is the perks you earn (even partial ones) can make these easier as the event proceeds, particularly if you can use several champions with perks in the event.

And remember, there are more champions will unlock, so with some cunning you can exploit that if you look up what the champions will do for you.

You aren't doing it wrong, just complete all three at tier 1 and tier 2 will unlock. Like the good ol' days but with limited killing power that is the best option anyway.