I posted this table awhile back, use the character editor to set the level to 99, and experience to 100 million, or even level 1 and 0 exp. The problem you experienced is probably because serena and veronica were both level 99 when you reached the world tree, causing this problem later when serena "awakens".

The cheat table is based on an old version (of the table, not the game) so just use it to set the level and exp, then save, then reload the game. 

Author: Nickd91

• Dragon Quest XI: Table for Cheat Engine (Steam) {lahma} [2019-May-10]
• Dragon Quest XI: Echoes of an Elusive Age - Trainer +18 v1.00 {FutureX} [2018-Sep-21]