Quick tips on Sledge (Lv18), especially if you're under leveled:
1. Bring a shock weapon to deplete his shields.
2. After that, use your most damaging weapon on him (it was a SMG for me).
3. Don't get close to him as his weapon HURTS.
3a. There's not a lot of room to navigate here and the area is somewhat dark. Some of the floor is grated and it makes it difficult to tell where you can run at quick glance.
4. There will be other bandits shooting you. Thin their numbers. You don't have to kill them all, but you don't want them all shooting you. I used the grenade mod that drains health and gives it to me and this helped a lot when I threw a grenade).
Keep that in mind and you'll be fine. He's not difficult, but he hits very hard on the Siren.
Tips for Siren gameplay:
Lure him to the hallway on the right side (as you enter the room) of the room after you thin the numbers of the other bandits. Sledge moves slow. Very slow. So run to the end of the hallway and start shooting him with your best weapon after depleting his shields.
When Sledge finally reaches you and corners you, your shields will likely be gone and you may be damaged. Use a Light health Pack and pop Phasewalk and run to the other end of the hallway and repeat. He should be dead by the time he gets back to you. Use grenades to thin the numbers of the bandits and pop a Light Health Pack anytime your shields are depleted, don't wait for your health to get low. Health packs restore health over time (10 seconds I think).
I went all Assassin on my skill tree for this fight. Added bullet damage and bullet velocity helps more than the skills in other trees.
EDIT: Here's another tip from https://www.trueachievements.com/Mr+Andy+Dixon.htm
. It's a great idea, especially if you're having problems killing a boss:
"Something that helps me with all boss fights is to leave a few of the lower level bandits alive so that if you get downed, you can kill one to get a second chance. Beats respawning and having to start all over!"