How much damage should a hard (not deadly) level boss do if I have 7 level ones

I also need an average health as I don't want my players steamrolling it or getting tpk

I assume you are Homebrewing? Find a CR calculator and run through the stats that you currently have in mind to find the CR, then create an encounter using XP guidelines in the DMG. Don't just make it one monster though - the encounter will be a lot more memorable and dynamic with minions to command, move around, and distract, plus it will last longer than the PCs dogpiling on some monster.

If you are running 1 boss vs 7 level 1 characters, you want to make sure you don't make it so powerful that if it acts naturally, it just kills one.

Example being if you give if 4 weak attacks, but there's only one PC nearby, it will whale on them and take them out - and that sucks.

Consider instead the challenge of the boss, not the deadliness of the boss. Realistically, low level PCs should start off against annoying enemies rather than outright deadly ones (hence their low hitpoints). Here are some example ideas:

• An AOE ability which deals 1d4 damage to everyone nearby as an action
• Attacks which grapple and move people instead of damaging then (check the Roper for inspiration).
• Swallow abilities (with risk of suffocation instead of acid damage - all characters can last 10 rounds, so it's largely an inconvenience as long as it doesn't run away)
• Abilities which knock people prone, or disarm them, or push them, making them think on their feet.
• Attacks which must target different creatures (see the T-Rex for the format for this).
• Illusion magic to make the players think "how do I tell which is the real one?".
• Ability to climb, to make the fight more tricky. Have it take place in an environment where there are tables etc. to jump from to get hits in.

Hope it goes well!

My bad, I misread the title and I thought these players were level seven. Adding on: don't let the monster focus too much on one character. Most level one characters are incredibly fragile, and an unlucky critical hit can instantly kill wizards and sorcerers through massive damage. Making a control boss that shapes the terrain instead of focusing on damage can be really fun with minions using this warped terrain to their advantage and doing the actual damage.