[nige1]

A friend asks "A regular partner of mine , Marc, is keen to explore the most thorough (as complex as you like) defense to 3 weak Acol 2's.
What would you recommend?"

I like a fairly orthodox defence, so I'm posting the question here.

[smerriman]

http://kwbridge.com/preempts.htm is a very good summary of the 'standard' defense, including leaping Michaels.

[DavidKok]

The above is excellent. As for complicated agreements, some suggestions that come to mind are the (I only know the Dutch name) 'Wereldconventie', which I'll just briefly give below (you could also google that phrase and pull the results through some translation software, but I think some of the suggested treatments in the top results are suboptimal), and some flavour of (transfer) Lebensohl for responding to partner's takeout double. For this last one I personally prefer the general outline described by Robert S. Todd in Adventures in Bridge (for a general introduction to these conventions, instead of a specific modern treatment, I always suggest reading the two articles by Larry Cohen on Lebensohl and Transfer Lebensohl). I combine this with a more detailed explanation of the transfer bids themselves, but this source (link) is only available in Dutch.

The 'World Convention' is used to show 2-suited hands in a single bid on the 4-level after an opponent pre-empts. It is 100% GF, but since it promises at least 5-5 these bids are often made on as little as 14 points (you often have to make these bids with around ~4 losers, in line with playing partner for a 'typical 7 point hand'). We play this convention after pre-emptive bids up to 3♠, as well as in specific situations (for example (1♥)-P-(2♥)-?), but the latter is not at all required.

If the opponents bid a long minor suit (2♦, 3♣, 3♦) a bid of 4♣ promises the other minor and a major. Partner can ask for the major with 4♦. You do not need this as a 'sign-off' bid, because partner holds a game-forcing hand. A bid of 4♦ promises both majors. This is a bit counter-intuitive over a 3♣ bid by the opponents, but worth memorising.
If the opponents bid a major suit, a bid of 4 in a minor promises that minor and the other major. A bid of 4 in their major promises both minors (this way partner can respond with 4NT or even 4♠ to show a maximum hand).

[Evies Dad]

I like the look of Transfer Lebensohl.

In general though, overcalling is easy with shape, and responding to partners t/o is also relatively easy with shape.
You can always choose whether to overcall with a less than ideal shape, so that is a matter of partnership style.
More problematic, and unavoidable, what does advancer do when weak to low intermediate with a balanced hand and is wishing partner hadn't overcalled ?

(2S)-X-(P)-?
(2S)-P-(P)-X-(P)-?

Do you have any special sequences initiating some kind of up the line scramble (depends on how pure your t/o is)

Are there any modern treatments that include a penalty element, like the original Hackett idea ?
(I don't mean when a t/o, often in 4th seat, is converted to penalty)