Downwell

You build combos by killing enemies without landing between the kills. Starting from a 5-hit combo, you'll notice a counter above your head. Every combo milestone will give you rewards, additionally to the previous ones: 8-hit will reward 100 gems, 15-hit an increased +1 bullet, and 25-hit a hit point/health bar.

The best place to get it is on Area 4-1 onwards. As there are almost no platforms to land, and tons of enemies and doodads to keep your combo rising up, you'll get this before Area 4-3. Keep in mind you can enter Timevoid caverns and shops without breaking your combo. Just make sure you don't exit them by falling over the edge, or else you'll cash in any reward and zero the combo counter.

Although not recommended, it's entirely possible to get the trophy in any other Area (I did it on Area 1, which is WAY harder).

The trophy will unlock when you land after building up a 100+ combo.