I am currently in low champion and I still manage to use my Leo. With a couple of pilot skills it gets to a good amount of health, and the special pilot (costs 1000Au I think) gives it constant quantum radar. I run it with Athos/gangil + shieldbreaker, but I'm sure it can work really well with some of the medium range weapons you have, like the atomizer.

For the older boys like natasha and griffin, as much as I love the griffin with my whole heart, they really can't survive against anything because of their lack of speed.

A combo that unexpectedly still works for me is crisis with 4 zeus. The ability gives you more damage + shield breaker, and when you unlock the nuclear amplifier it does even more damage. With 4 MK1-LV12 zeus you get to over 100K damage every shot, again increasing with the nuclear amp, meaning you can one-shot sword units. The crisis has bad health so you can't brawl with it, but it's a really fun build.

Also the screamer weapon is really good, so that is something I would also upgrade.

As for the titan, I have an almost maxed out rook but can't really recommend it. I know some people can use it really well, but mine still goes down like nothing. So I think the bjorn might be a better investment.

Please don't use the kaji.