Double Dragon 4 (UP)

Tower mode is unlocked after completing the main story for the first time.

The recommended character for this achievement is MysteriousWarrior, unlocked as playable character after beating 40th floor of tower mode.

Here are some tips for getting to 100th floor with MysteriousWarrior:
1. Use top and bottom borders at your advantage. As it happens in story mode, enemies cannot attack you if you stick at the top or bottom border of the fighting area. Be aware that sometimes and for no particular reason enemies can attack you even if you are in such "safe zone", mostly the bottom one: if you notice some space between the border and the feet of your character, quickly switch from bottom to top border.

2. Projectiles (hadouken like fireballs) do regenerate every 5 floors, so use them quite often. A single projectile can one-shot minor bosses like Abobo and Burnov.

3. Use "spinning arms" special move as your main attack. Since enemies' health is slightly smaller compared to story mode, you can basically spam the spinning arms special through all tower mode, ignoring basic attacks. If you time spinning arms correctly, you can even use it to dodge projectiles.

4. DO NOT rush, take your time. While tryin to reach the 100th floor, I died 75% of times because of rushing through enemies. Even lesser thugs are dangerous if you get stuck between two or more of them so take your time, walk around the "safe zone", always try to isolate your target before attacking it and be careful when you are in crowded situations.

5. Hardest floors are 30, 40, 75 and 80, so pay particular attention while there. The combination of Sonny spamming fireballs and MysteriousWarrior (on 40 and 80) can literally kill you in a couple of hits.

Here's a video by cenorexia demonstrating how to use MysteriousWarrior to get the achievement. Enjoy :)