How do you tackle, multi-generational marketing.......YOU DON"T....it doesn't work and its never going to work.
I'm sorry, but any marketing professional trying to build a strategy around those two generic labels, needs to find a different line of work.

First the age ranges in those categories are too broad to be useful for targeted marketing, even at a grocery store, that large a demographic isn't going to have similar shopping patterns, you're litterally talking about a gap that covers college students to senior citizens.

Boomers spend more than millienals well no crap considering the "Boomer" age range is between 51-72, you have a group near retirement, in retirement or frankly sitting around waiting to die. Of course they are spending off their assets, you can't take it with you and they've already made all the money they are ever going to earn.

Let's see millenial age range would be 23-39, too broad, but ok, so you potentially have new college graduates/grad students to adults well into their career. So no kidding, a large portion of that group isn't even working yet or is at their first job, so of course they are not going to have as much money/assets as the boomers e.g. their parents/grandparents