Yes Decrep is the ‘safe’ curse for those who feel they need to slow things down to control the area better. Amp does better for damage when you don’t feel you need to slow things down.
Breaking it down:
Amp Damage lowers physical resistance by 100
Decrepify lowers physical resistance by 50 and slows the target.
As you can see Amp does more damage increasing your killing speed etc…There is a comfort level when running a summoner where you are no longer concerned that your army will be blown through by whatever you are fighting. At that point you don’t need to Decrep, you can amp and be totally safe. Part of this is what level your skellies are another part is just how comfortable you are.
Immunities: lower resists works on a 20% efficiency for immune monsters. What this means is Decrep will lower the immunity by 10 and Amp will lower it by 20. If the monster has resists of 100 Decrep will break the immunity, if it has resists of 110 Decrep will not work, you need Amp to do the job. At 120 resists you can’t break it and you need a different source of damage.
My recommendations for using Decrep: Starting off with Bosses: you decrep them. Between Decrep and Clay golem you slow them down so nothing hurts your army.
If you have lvl 20-25 skellies you probably want to decrep Lister’s crew. Your skellies may not make it through them and your better safe than sorry. As your lvl of skellies goes up you don’t need to worry about them as much. Frenzytaurs can use some decrep as well.
If you don’t have a Dim vision at about lvl 10, you can use Decrep to slow down the piercing kitties, gloams and leapers. I prefer Dim for them as it shuts them down but slower is better than full speed. I personally have lvl 13 curses with just 1 pt invested (trangs gloves, Blackhand Key and +9 to all necy skills.) At very high end gear you can get +18 to all skills easily giving lvl 19 curses with 1 pt invested.
For just about all other situations I perfer Amp, but I also use a controlled chaos method of playing…I like the chaos.
Spirit for a Necy is generally not needed. It works if you are buffing with CTA + Spirit but by that time you are close to getting Enigma and that will give you the ST you need to get the Monarch equipped. Shrunken heads will come with the +2 to skills that you want while running the necy. I have a rare shrunken head with +2 necy skills at the moment. Homu is better in most measurements, better resists, block rating, mana per kill, mana recharge etc. Spirit can help you hit telly breakpoints but if you are using telly that much you are bypassing things, you can just use the CTA/Spirit on switch and then move to Homu/other for fighting…
You will probably be able to make a Spirit long before you find the rare shrunken heads to Homu though.