Infuriating trash war question

Background:

Recently came back to the game after 15+ years not playing and convinced a buddy to switch from sc2 to AoE2. ELO rating noob.

Problem:

He always plays Slavs and always just makes it to Imp before budding into a full fledged trash war monster. 15-20 barracks minimum. He always goes Onager + Halb (Druzhina). I've tried Byzantines (Logistica), Teutons, Aztecs (Jaguar + Garland), Magyars (Huzsars to counter siege and Arb to counter Halbs), Celts (mix of Woad + Rams) to get into his barracks .

Solution?

Assuming game develops into post-imp, thought about Britons but too much spam from him and not strong enough micro strength from me, and, Chinese but Chu Ko Nu fully upgraded only has +1 range on Onagers. Turks in the mix as well but somehow not the biggest fan.

Any advice?