Welcome to Redemption at Gontranno, both the most annoying and most awesome level in one. Please note that you won't get a rating for this level, meaning there is no way to obtain Silent assassin. Going in guns blazing is the only way to go here.
The annoying part
The single greatest annoyance of this game is the start of this level. Fortunately for professional difficulty this means you can restart without losing a lot of time until you get it right. The problem: You are unarmed and there is no way to get out of the first room unseen without taking out a guard. The first thing you will need to do is acquire weapons.
Go directly right after both guards disappear behind the pillar and crouch. One guard (1) will walk fast and make a round around the place, the other (2) will patrol in front of the door. You cannot sneak past this guy because the guard (3) behind that door will always spot you when (2) doesn't and visa versa. I've tried several times on normal difficulty, there is no opening in their routine and you only have a limited time window before (1) sees you. And going through the church doors will result in you being gunned down by a guard (4) with a machine gun.
You can do two things: Make a run for the garden shed and arm yourself and pick up the first weapons you see as you will have to fend off several enemies immediately, or take out your fibre wire and sneak up on (2) before he turns around. This is a very, very tight time window so it's advised to do this in 3th person and crouch backwards (because you'll go faster for some reason) before strangling him. Take his gun (don't bother with his uniform, the guards won't be fooled for even a second.) and run to (1). If you put a few meters between you and the church, you can gun him down without (4) hearing you so he won't find (2)'s body. Then, kill (3) and the guard patrolling below. It is possible to do so without alerting the others but not neccesary.
Make your way to the garden shed, take a drop rather than the slope so the guard with the sniper rifle won't shoot you, and arm yourself. Pick up a heavy weapon. If firepower is what you want, the M60 is recommended, but I personally think that if there's ever a time for the Ballers SD, this is it.
Make your way to the church. Kill all the enemies, this is possible to do unnoticed with the ballers SD until you get upstairs. Advised: go upstairs and gun down the man behind the left door before taking out the two behind the right door. Ballers will send the first guy over the railing but are still advised because they can launch the SVD towards you and (might be a bug) can launch one of the other two guys against the locked door behind them so hard it'll open. Take the SVD or go back to the shed to collect it and shoot the other man on this side of the balcony, whose carrying a 1shot kill custom rifle before he can shoot you. Also shoot the other two guys on the opposite balcony as well before grabbing the custom rifle. Use it to snipe all the guards below (can be done unseen) before making your way back downstairs.
Walk into the confession booth, shoot through the heart and when Sergei emerges finish him off. Ballers can break the heart with 1 shot while the custom rifle needs more shots, depending on whether you skip the cutscene or not, Sergei will break through the front or take the door behind him. Once you kill him, you win the game.