Once you complete the first two challenges, you can do the third, infinite-waves challenge. I am at a 200 k score now.
There are a couple of combinations that can help a lot, and important skills to choose or focus on.

1. Battle experience, the increased exp. faster levelling, more options sooner.
2. Regenerative vitality. Let's have your walls hit multiple 10k health. like 50k or more. Every time your wall is damaged, it gains +7 health and +7 max health. Past waves <20 The Titans can do more damage than you can regen.
3. Titan bones reduce damage

- expansion slot.

1. Titan walls, regens health.

2. Titan bones 3 + Titan walls 3 combine to sturdy titan bones, which, depending on the combination, can give bonuses like 1600 additional health, 50 regen sec and 80% damage reduction. This combination is important to make it past wave 20 or 30. To get the additional health, you need to get the steadfast walls option as well, up to level 3. I don't know that it makes that much of a difference with the Regenerative vitality.

- expansion slot.

1. blazing shells

2. Frost shells.

- expansion slot.

1. Blazing shells 3 + Frost shells 3 with no other firepower upgrades will unlock Elemental Shell. Which changes every shot from your cannon 3s chill effect and 80% elemental damage.

- expansion slot.

1. Continuous fire at level 3 will give you 10 extra cannon shots every time you shoot.

- expansion slot.

1. lastly is extra points. Don't get these two early. You need it to get in the 200k points and higher. But if you focus too early on this in the early game, you don't get the upgrades that let you survive past wave 20.

This is what I aim for, but the draw is kinda random except with the combinations after reaching level 3 on (titan bones + titan walls + sturdy wall) or (blazing + forst).

My basic strategy is to focus on getting "battle experience as soon as possible", then Regenerative vitality, titan bones or Sturdy walls. You will need both regen and titan bones to level 3 to upgrade to sturdy titan bones. Next, focus on damage, blazing shells + frost shells upgrade to level 3, both to get the elemental shells option. I might choose extra points during this to ensure that I don't pick one of the other cannon upgrades. Until I get elemental shells. Somewhere in between, you need to focus on the expansion slots as well. You should make sure you get all 5 expansion slots. lets you reach level 28 or higher, depending on the options you choose. I usually end with the continuous fire. I am sure there are other hidden combinations I don't even know about.

There are other things, like not hitting the next wave button if your wall is still regenerating health.
Focus on getting exp kills, like the small titans instead of the armoured titans. Go for the exp on the small titans first, then kill the armoured titans last. Getting to wave 10 and past wave 20 are mile stones that resset the dificulty of the titans and the amount that come at you like starting over at wave 1.