I think that guardian zilean (max E first) doesn't work very well in low elo. I won more matches using aery or glacial and building ap. (sorry if my english is bad) :
Build VS FULL AP/AD team as support zilean : r/ZileanMains
LoL Guide, Zilean, Mid, S10 - Millenium
WTF?! FULL AP ZILEAN BOMBS DO HOW MUCH DAMAGE?! (THIS IS HILARIOUS) - League of Legends - YouTube
This is my usual Zilean build. I know the items and runes are somewhat unusual but this build carried me a far way and it works just fine for me. Items are
Zilean Build Guide : How to carry as support | Support Zilean :: League of Legends Strategy Builds
Zilean | League of Legends PH Wiki | Fandom
Zilean Middle Build, Runes, Counters (Patch 13.08) - OP.GG - League of Legends
ZILEAN FULL AP MID - League of Legends [PT-BR] - YouTube