I found that Lazar was really hard to bet, mainly because he finds loads of ways to cause
effect damage and stop you attacking him back.  He uses cards like Stealth Bird which
cause you effect damage when they are flip summoned, Giant Germ which cause you damage
when you destroy them and then summon more of them, Dark Room of Nightmare which causes
you more effect damage when you take effect damage, and, the really annoying one, Lava
Golem, which he can use to get rid of 2 of your monsters before causing you effect damage
while on your side of the field.  However, I found that the best way to defeat him was a
mix of negation and penetration.  Because he uses only one high ATK monster - Caius the
Shadow Monarch (2400 ATK) I use monsters which have a penetration effect, like Spear
Dragon. However, he would counter your attacks with cards like Mirror Force.  So, I always
have traps of my own to stop him from doing this, such as Trap Stun or Royal Decree (note
if you are using Royal Decree, use less trap cards in your deck).  His monsters tend to
have low DEF (except for Stealth Bird - 1700 DEF) so when you cause him penetration damage
it is generally quite high.  A technique that I used was to have a trap negation card on
the field, then I used Five Headed Dragon equipped with Big Bang Shot to cause the
penetration damage.  A technique which I could advise would be to use a number of trap
negation cards, while using cards to empty his hand and deck (I would recommend emptying
his hand if you are going to attack hard because he has Lightning Vortex, which will
hammer your attack).  A technique which Lazar uses is to summon Jester Confit, which can
be special summoned when he has no monsters.  Then he will Tribute Summon Caius the Shadow
Monarch.  Now, because he used Jester Confit (a Dark monster) to summon Caius, its effect
allows Lazar to remove from play one monster on your side of the field, before causing you
1000 damage.  I also used the card Mirage Dragon to stop him from activating Trap Cards in
the Battle Phase, which stops him from using cards like Mirror Force and Nightmare Wheel
to stop your attacks.  Another card which could help is Magical Cylinder, which could be
used to reflect the damage that Caius tries to cause.  A card which Lazar also uses is
Waboku, which he uses as a last resort supplement to Mirror Force.  If you successfully
negate Mirror Force, he will almost always use Waboku afterwards unless he can stop you
negating Mirror Force.  He is good at negating your effects as well.  When I finally beat
him it had a panicky finish.  He had 2300 Life Points left, and I had 2 monsters which
could take them away (Spear Dragon and Flamvell Grunika - 1900 and 1700 ATK respectively)
but I had been beaten like this a lot of times before when playing against him, so I had
negation trap cards ready.  I attacked with my Flamvell Grunika, and, surprise surprise,
he activated Mirror Force.  Too bad for him I had Trap Stun to negate his trap.  Then he
activated Dark Bribe, which would cancel out the negation effect of my Trap Stun.
However, I also had Dark Bribe, and used it to negate the effect of his Dark Bribe, which
also meant that Mirror Force remained negated.  I had predicted his Dark Bribe, which was
why I activated Trap Stun first, so that my Dark Bribe would be used first in the chain
(Trap Stun negates the effects of all Traps that turn), so it wouldn't be negated by Trap
Stun.  This meant that the Flamvell Grunika hit took 1900 Life Points away, which left my
Spear Dragon to win the Duel.  It took me a long time to beat him, just keep trying until
you get lucky.  The luck I had was when he played really stupidly.  He summoned Lava Golem
to my side of the field, then used Lightning Vortex to destroy it, when he could have just
used Lightning Vortex initially and saved Lava Golem for another time.