A thief will initiate a fight with a basilisk venom/backstab combo. The initial hit will be roughly 10k HP.
A bit of advice on this part: That initial hit is what you need to learn to avoid. And if you do avoid it, the rest of the fight will be greatly in your favor since he’ll be down an elite and half of his initiative, and a large chunk of his hp if you play your cards right.
Basilisk shows up as an icon on his status bar, so pay attention to it. If you see a Thief coming at you with the basi buff, that’s your cue to prepare for evasive maneuvers.
Pay attention to range as he approaches. 900 is the default range of Steal and unless he has a Infil signet (which you can also see on his bar) or traited Steal range (not many people run with this), this is the maximum range he can begin engaging you at, and most Thieves do tend to initiate at the max range possible. Keep target on him and pay attention to the range indicator on your skill bar and note when he gets within the 1200 range of decoy or mirror images. It’s hard to judge when exactly to start countering against D/D bursts since Steal is instant, but most Thieves usually precast C&D before stealing, so if you can see him raise his left arm while still at range, that’s usually a good cue to start countering.
When it comes to counters, here are some that I recall Shatter Mesmers using against me to a great effect (by no means exhaustive, ordered from highest to lowest effectiveness):
1) Blurred frenzy. This will make the Thief Steal into one of your hardest hitting, and hardest to land bursts and eat most of it, while you remain completely untouched. A bit of risk and reward at play here. If he doesn’t burst into your Blurred frenzy then you’d be down a skill for ~10s, but even then most Thieves would likely try to time his burst for when your Blurred frenzy ends, so you can chain it with one of your other options.
2) Illusionary Riposte. This will block the Steal and punish him with another very hard hitting burst. And teleport you away from where the hit landed and generate a clone there to absorb the C&D follow up.
3) Decoy. This will tend to make most Thieves steal and burst into the clone. The invis will buy you time to plan for a counter, but it doesn’t directly punish the Thief, and is a valuable stun breaker so I’d only recommend this as a follow up if the Thief waits out the above two.
4) Dodge. Again to reiterate, it’s hard to time dodges against D/D bursts since Steal is instant, but most Thieves usually precast C&D before stealing, so if you can see him raise his left arm while still at range, that’s usually a good cue. Dodging will leave a clone to confuse the thief and combined with mirror images can be used to land a full powered shatter with almost 0 delay.
5) Distortion. Although it works, this isn’t a good way to counter the initial hit because you’d have no clones out, wasting up to 3s of invul time. A better time is to use it mid battle when he Stealths off one of your clones to nullify his followup backstab.
Note that most of these tactics can also be used during the fight if the Thief doesn’t initiate with Basi. Since Basi has a cast time, you’ll be able to see it coming if he casts it out in the open, and you’ll be able to use any of the above to make him waste that Basi charge. More troublesome is when he casts it in Stealth, but you should be using one of the damage mitigation options above every time he goes into Stealth anyways to avoid the follow up Backstab.
