UNDERTALE: List of Boss Characters and Order of Battles

Boss Monster List

Monster	Location / How to Spare
Toriel	【Location】 Ruins (All routes) 【How to Spare】 Keep selecting "Spare" while avoiding attacks
Papyrus	【Location】 Snowdin (All routes) 【How to Spare】 Keep selecting "Spare" while avoiding attacks
Undyne	【Location】 Waterfall (Neutral Route/Pacifist Route) 【How to Spare】 Select "Flee" when the soul is red and keep fleeing until Hotland
Mettaton EX	【Location】 Core (Neutral Route/Pacifist Route) 【How to Spare】 Avoid attacks and raise the ratings to 10,000-12,000+
Asgore	【Location】 New Home (Neutral Route)【How to Spare】 Cannot be spared
Flowey	【Location】 New Home (Neutral Route)【How to Spare】 Cannot be spared
Asriel	【Location】 New Home (Pacifist Route)【How to Spare】 End the battle after a certain number of turns after reviving all NPCs including "Someone Else"
Undyne the Undying	【Location】 Waterfall (Genocide Route)【How to Spare】 Cannot be spared
Mettaton NEO	【Location】 Core (Genocide Route)【How to Spare】 Cannot be spared
Sans	【Location】 New Home (Genocide Route)【How to Spare】 Cannot be spared

List of All Monsters' Sparing Methods

Mid-Boss Monster List

Monster	Location / How to Spare
Napstablook	【Location】 Ruins (All Routes) 【How to Spare】 Cheer 4 times
Doggo	【Location】 Snowdin (All Routes) 【How to Spare】 ・Pet ・Use Stick
Lesser Dog	【Location】 Snowdin (All Routes) 【How to Spare】 ・Pet ・Use Stick
Dogamy & Dogaressa	【Location】 Snowdin (All Routes) 【How to Spare】 ・Make one of them "roll around" → "re-sniff" → "pet", then pet the other one ・Use Stick
Greater Dog	【Location】 Snowdin (All routes) 【How to Spare】 - "Beckon" or "Pet" -> "Pet" -> "Play" -> "Pet" -> "Pet" - Use Stick
Shyren	【Location】 Waterfall (All routes) 【How to Spare】 Perform the action "Hum"
Mad Dummy	【Location】 Waterfall (Neutral Route/Pacifist Route) 【How to Spare】 Keep hitting with magic attacks
Royal Guard	【Location】 Hotland (All routes) 【How to Spare】 "Polish Armor" of RG02 -> Touch the green armor 5 times -> "Whisper" to RG01
Muffet	【Location】 Hotland (All routes) 【How to Spare】 - Use the item "Spider Donut" or "Spider Cider" - After 17 turns, you can spare her
Memoryhead	【Location】 True Laboratory (P Route) 【How to Spare】 - Use "CELL" on one of them, then use "Refuse" on another
Endogeny	【Location】 True Laboratory (P Route) 【How to Spare】 - "Beckon" -> "Pet" -> "Play" -> "Pet" -> "Pet" - Use the "Stick"
Lemon Bread	【Location】 True Laboratory (P Route) 【How to Spare】 - Select "Hum" -> "Flex" -> "Un-hug" in any order
Snowdrake's Mother	【Location】 True Laboratory (P Route)【How to Spare】 Choose "Joke" 3 times
Reaper Bird	【Location】 True Laboratory (P Route)【How to Spare】 Choose "Pick on," "Mystify," and "Pray" in any order

List of All Methods to Spare Monsters

Boss Strength Ranking

Rank	Character	Evaluation
1	Sans	・G Route Final boss ・Uses a special move at the beginning ・Takes time to get used to the battle ・Cannot hit Sans until he falls asleep
2	Undyne the Undying	・G Route Transformed version of Undyne that appears ・Attacks significantly stronger than regular Undyne ・Takes time to get used to the battle
3	Flowey (Photoshop Flowey)	・N Route Exclusive boss for the first playthrough ・Cannot use items and limited healing options ・Launches one-sided irregular attacks
4	Asriel	・P Route Final boss ・Becomes invulnerable in the second form ・The strongest monster in terms of stats
5	Undyne	・Boss of the N and P routes "Waterfall" ・Has a high intent to kill the player and launches multiple spear attacks ・Undyne's strength is the strongest among monsters

Ranking Criteria

The ranking of boss strength is determined primarily based on the difficulty in avoiding attacks and the amount of time it takes to defeat them.

With the exception of Mettaton NEO, bosses in the Genocide Route are highly regarded due to the high difficulty level of completing the Genocide Route storyline.

List of Story Walkthrough Charts