Why engage him in a risky spot, much less tar pits? I mean you can do the brute force Dark Souls approach for the challenge but you're making the game harder than it is.

Just move AWAY from the tar pits and kill him. And keep your shield in block mode.

And if really want to play it safe, move him to a spot that you know is completely safe and has your portal close by (e.g. on a big rock you can jump to or over a pit or whatever) by so you rest, recharge whatever god power you want, return and beat him whenever you feel like it.